Just had the same issue with my 935. I think it stopped working last winter but I just took forever to do anything about it. Well out of warranty but Garmin replaced it anyway no questions asked. Rather not have the issue in the first place of course but can't really beat the customer service.

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Just like you, the altimeter in my 935 has died...twice.

Mine died a couple of years ago and I've just lived with it ever since. I'm hoping to get a new watch in the next few months. I'm looking forward to not running at 50,000 all the time

I had the same exact issue with my 945. Altimeter failed inside of 1 year but they were able to send me out a replacement. They claimed it was from use with swimming.

You can see the advertisement now.
"Garmin. THE best Ironman watch for all those swim-cancelled races."

yeah, the triathlon watch that you can't swim in!
interestingly, my 935 (the 2nd one after the 1st suffered this fate) failed shortly before lockdown. a month or so out of the pool and it worked again. back swimming regularly for 2 months now and still working. it was officially a dead barometer according to the garmin diagnostics that i know about from my first one so it seems it is true that it is an issue with water clogging the barometer port
i imagine the refurbished unit they sent me to replace the first one was someone else's post-swimming failure which just needed a break