Good afternoon..., I have a naive question. I have had my carbon frame since 2021, and I have never used a chainstay protector.I have noticed that in that part of the pod, there are paint splashes. I'm scared that the frame will break. I use it on the road, never other terrain. Is it possible for the frame to break if I don't use a chainstay? Is it too late?
Here are some pics.

I see cosmetic scraping of the surface (gel?) coat and nothing more. Will be surprised if there's structural damage, but happy to defer to the CF experts. IMHO the bike is perfectly fine.

You could place protection over the area--clear helicopter tape or something more visible--now that you see where you're getting chain slap. Should prevent any further marring.

CS protectors are mostly for the cosmetic scratches and chain slap chips. I've seen enough Trek stainless CS protectors that were gauged and torn up after a serious chain suck incident and didn't prevent some carbon damage to believe they do their claimed job well. So I would rather trust my good skills at shifting to keep from damaging my frame then relying on a lightweight thing that is weaker than the underlying carbon is. Andy

It's unlikely that chain slap will do worse than cause cosmetic damage to the top of the stay, that's what a protector guards against. Chain suck OTOH can gouge chunks out of the chainstay, as seems to have happened very slightly on the outside of yours. There's not much you can do about that, in the worst case the chain will go straight through helo tape; just keep your transmission in good condition and well adjusted. I sometimes wrap a chainstay in rubber (old inner-tube cut into ribbon) as much to quieten the chain slap as to protect the frame, but I don't care much about paint chips, because I don't own carbon bikes.

It's never too late to add a chainstay protector of CF frames. So far you've only seen cosmetic damage, but that can change.

FWIW - chainstay protectors fail to protect the most critical place, which is right by the chainring. Chain suck or something similar can jam the chain up against the stay with the gap being too narrow to let it pass. This can cause serious local gouging, which a simple plastic film cannot prevent. It's a pet peeve, but IMO a ballistic plate, ie. shaped stainless steel, should be bonded to chainstays at this very critical location, to ensure the chain cannot cut into the stay.

https://rideyourbike.com/carbonfiber.shtml

At the very end of that web page (warnings against carbon fiber), there is a pic of a wheel reflector driven completely through a carbon chainstay. So my view is, some armor there would be a plus.

I agree, with one suggestion: So steels have a lot of toughness, so can distort a lot without puncturing. However, that distortion could still damage the chainstay underneath. I think what would be desired is stiffness, to distribute an impact force over a much larger area. Accordingly, I would recommend high-strength aluminum, 3X the thickness for the same weight, would be good armor. Stiffness is a function of the cube of the thickness, for given material properties. Aluminum is only 1/3 as stiff as steel, but 3X thickness cubed, is 27X, divide that by 3 for less stiff material, and you still get 9X as stiff.

I worked for a time in military vehicles, including armor. Sometimes you need ultimate strength, sometimes you need stiffness, sometimes you need hardness. Often combinations of the above. But it often takes different approaches, based on what is immediately behind the armor.

Possibly you misunderstood my intent.

I don't see my job here on BF to include detailed designs and specs. If it were, I'd expect to be paid a consulting fee.

The reality is that the ballistic protection I described is properly the job of the frame's maker rather than the owner. As a practical matter, designing, spec'ing, fabricating, heat treating, and fitting that part is beyond the capabilities of end users.

I was simply pointing out a vulnerability as something to consider in choosing a CF frame. Too often, folks get distracted and worry about the wrong thing.

Understood, and agree.