After 9 months of riding with a 10 cm stem (decreased from the stock 11), I want to increase the comfort. It looks that my available time for training will reduce, thus, I can not face the stretched ("race") position anymore (although in the near past I was able to make 2 rides of 150 km with that configuration).

I was wondering whether a further reduction of 2 cm (stem lenght going down from 10 to 8 cm) will make a 59 size bike to look strange, or is it still acceptable?

I ride a 58 with a 90mm stem. No one has ever mentioned it. The bike is very comfortable for me. My other bike is a 55 with a 100 mm stem.

I have my doubts. Here's my 61cm CX / commuter bike with an 80mm stem.

its ok.
basically what you are doing is creating your own version of an endurance bike sized 57 or so in reach and 59 in height. There is a reason they are so popular. For a given reach, many prefer a taller head tube to increase handlebar height based upon lack of flexibility and ride in a more upright position.

Yes, consider also adding spacers under the bars if you have enough steerer.

As for how things look ... everyone sees the same things differently. One thinks it's cool, another thinks it's stupid, and most don't even notice. If you want to enjoy riding your bike as opposed to pleasing some imagined bike-appearance Nazi ... then most people will see first and remember most your big smile.

You know how fads start? Some guy has the courage to do what he thinks is right, for all the right reasons ... people see his confidence and copy him, for no good reason. Start a fad .... short stems will be all the rage for a while ....

If you don't want to swap the stem, you could swap handlebars to a 'short and shallow' style and get the same decrease in reach, at least at the hoods and drops. I find it funny that these bars are popular yet short stems are frowned upon. Both affect steering inputs from the most common hand positions yet it is only the mention of short stems that brings out comments about 'awful handling'.

If my kid can handle a 43mm stem on his bike, an adult should be able to handle an 80mm stem.

Acceptable to who? Who cares? Use what works and don't worry what someone will say when you post a pic on BF.

More than appearance I would be concerned about handling ramifications from your ever-shrinking reach. I enjoy the way bikes with exceptionally long stems handle. It seems almost more deliberate in the way the bike steers. Very short stems can feel twitchy, especially at slower speeds.

From 10 to 8 cm, the handling of the bike will be a lot more sensitive. The center of gravity also changes. Take some time to get used to. I probably wouldn't do switchback descent immediately after the switch.

You can always spot bros with tiny stems on too-large of a frame because they are swaying left and right when working hard, climbing, or sprinting.

I spoke too soon about the handling comments. They all arrived after my post about how the handling change is a non-issue

And guys...when I was young and a student of cycling and trying to figure out my best fit, I wrote to Sheldon Brown for advice and asked about riding a 60cm bike with shortish stem and he was gracious enough to write me back in his profound style and say...
anything in the 80-120mm range is perfectly acceptable on a large or extra large bike. He went on to say that humans are extremely adaptive creatures and any effect of shortening the 'tiller' would be quickly adjusted to in terms of handling. He went on to talk about owning a wide variety of bicycles from all over the world in all shapes and sizes including a 60cm bike with short stem. He was just over 6' tall like me and one of the reasons I sought the great man's advice. He used the analogy of owning a tandem and how it handled versus his shortish wheelbase racing bikes...the miles melt away any significant difference in handling.

A good rider will be unaffected. In fact, since many geometric aspects of any bicycle affect handling as much or more than stem length from wheelbase to trail, a short stem on a longer wheelbase bike may quicken the handling of a bike to be construed as a positive.

I run 90mm stems on all my drop bar bikes which have (in order) 57, 58 and 59cm frames. I just like that length, as it turns out, as the bikes have a variety of stacks and reaches, but I'm almost always more comfortable on them with a 90mm stem.

Nobody has ever made a negative comment about my stem length.

I have a 54 size road bike with a 50mm stem. Handling for me is fine.

Some people say they prefer a larger frame with shorter stem, because of the bigger wheelbase, making for a more stable ride. Others say the steering is too "twitchy".
So basically, it depends who you ask.. I've always been fine with it. I definitely feel as though my wheelbase is extremely stable.

As for looks.. yeah it looks a bit weird from the side to have such a short stem, but from the front or rear you cant tell.

I think a 80mm stem is still not really THAT short. It will be fine! Comfort is more important than how it looks anyway.

Agree with Lazyass that it shouldn't matter what it looks like as long as you're satisfied.

Also, remember there are a lot of women's bikes with stems 80 mm long or even shorter in addition to narrower bars which works tend to exacerbate any "tiller effect". The bigger issue could be weight distribution, i.e. less weight over the front wheel. But it's not a huge change in length. My bet is that you'll be fine.

I wasn't comparing pros and cons of shorter/longer stems on handling. The point I was trying to make was, from longer to shorter stem the handling could be a lot "different". I've changed saddle to handle bar drop, saddle, saddle height, seat post, wheels on the same bike....Based on my own experience, nothing affected handling as much as switching from 110 to 80 mm stem.

Very interesting article:
How does stem length affect a bike?s steering and handling? | CyclingTips

They sit upright so a larger portion of their weight is on the rear wheel too. And they constantly ask why they get wobbly going downhill with their but planted in the saddle, their elbows locked tight, and their head straight up like a frightened turkey.

I just ordered a 90 cm stem to replace a 100 mm stem, will that do much to performance?

I run 90-110, with the 110 having enough rise to basically keep the same length as a flat 90, so I get more rise while keeping the ETT or reach the same. Nobody has ever mentioned it and I prefer larger frames to get more rise plus no toe overlap. I technically would fit a 56 but no longer buy 56s. I buy 58s. I have plenty of stems so can adjust as my flexibility or fit needs change. I think it is fine. I have a friend who (after a fit) actually uses an adjustable stem. That things looks pretty weird but nobody has said anything. People rarely notice these things, and what does it matter anyway. It isn't their body or their bike.

Have you ever tried a short reach handlebar? Or moving the brake levers up higher on the bars? Either will do the same to handling as a shorter stem and are pretty well accepted by cyclists. But somehow short stems are always a 'major' handling change. A change, yes, I agree with that. But nothing major and no different than other changes that no one warns about.

You'll lose at least one Strava KOM. Whether you can accept that or not is quite personal. You may gain it back by deflating your tires 0.5 psi.

I have 120mm -17deg on a road & CX bike both 51cm, 110mm -17deg on 53 cm and they all feel great. I've done 160 & 200 miles on both road bikes with no issues.

2 weeks ago, I went from a 110 mm stem to an 80 on a 57cm bike.
Feels great! Looks fine. Do it.