Is it true that once I open a 10 speed chain, I can't close again without buying some part?

It really depends on the chain. Shimano and Campagnolo both require a special use once pin. Sram, KMC and others use connectors. Some are reusable, but Sram for one says not to reuse. OTOH- many report reusing Sram 10s links successfully, so that's your call. You can also use reusable connectors on Chins that otherwise would use a pin.

BTW- with connecting links you reopen the chain at the same place each time, but for Shimano and Campy, leave the 1st closure pin in place, and cut the chain in a new place. That's because you don't want to keep stressing the same hole pushing the pin in and out.

My preference is to go with the connectors; they are called various names, master link, quick link, power link, etc. You need to match the size of the link to the chain; 7-speed, 9-speed, etc. The connectors can be opened and closed without a special tool although tools are available and are convenient. You will still need a regular chain tool to cut a new chain to the proper length. If you decide to go the one-use pin route they must also match the chain you are closing. Buy a few, you will need a new one each time. As mentioned above don't open the same link twice.

SRAM technically can't be reused, but I have with no issues (so far), but prefer KMC connectors, they are expensive but worth it. No problem with using the Shimano pin method either, just works out even more expensive than using KMC connectors.

Any thoughts on opening and closing old fashioned way? like removing a pin halfway trough and then drive it back? - I admit I have not used 10speed yet. but for 8speed and single speed old fashion driving the pin (any pin) back, works just fine.

Yeah, don't.

pressing pin out stretches the side plate hole, thereafter that is weak.

if possible re riveting the pin head will snug it somewhat..

but the chain making machines do all the pins identically.

not sure if a Campag, or Rohloff chain tool,
that may be of some help, can match it..

+1 on don't. Even 8-speed chaind are badly weakened by pushing a pin part way out and reusing it. These narrow chains have their pins "riveted" at the ends for strength and pushing them out removes the riveted flair and reams the hole oversize.

Most definitely don't try on a 10s chain.

the difference is that the pins on your chain are flush to the rivet so there's no safety margin if the plates spread. To compensate, ends of the pins are peened into a small countersink in the plate forming a rivet head (the same way bridges are riveted together). When you press the pin through you shear off the head, so when you push it back you have no defense against plate spread which will lead to chain breakage.

Will it always break, probably not, but it's likely enough that you won't like the odds, especially if you tend to shift aggressively while under load.

Thanks for clarification. I did not know that all 10s chains are riveted. Some of 8sp are, some don't. The ones I've had were without deforming on the end of the pin or something with a master link of some sort.

I agree. that's what I regularly do (3 different chains so far this year) On one of the chain I did last winter I already have 2800 kms with no problems at all

Unviversal Cycles sells reusable (for the same chain - you're supposed to replace the link at the same time as the rest of the chain) 10 and 11 speed KMC missing links for $3 and the wider ones for $2 each.

https://www.universalcycles.com/shopp...93&category=59