I'm somewhat active on r/FirefoxCSS, and have substantially customized my Firefox UI. Never saw any deprecation notices in the console, or a message on the subreddit.
I highly doubt Firefox would ever drop support for userChrome. The themes aren't nearly as powerful and low level. The about:config toggles works as a footgun protection. I have yet to see any technical reason for them to drop support.
That said, Firefox has dropped support for small yet useful features for no apparent reason, so there is a non-zero chance of them dropping support for userChrome customizations.
Killing userChrome would be extremely on brand for Mozilla, so I'm sure it will happen at some point. Killing features beloved by a subset of power users appears to be a guiding design principle there. I've seen userChrome suggested as the fix for all kinds of UX regressions that Firefox pushed on users. But if Firefox is willing to go out of its way to break the UX in various ways, why would I ever trust it to maintain that support?
Of course people are going to find ways around it, simply because the alternatives are still so much worse, but it definitely disappoints me to see that we have to be fighting this constant war with the developers at Mozilla who are continuing to oppress users while thinking that they're doing the right thing.
My company hired a senior developer for an Angular project we were working on. They did great in the interviews and on our take home coding challenge. When it got down to time to work, I was walking them through the codebase and it very quickly became apparent they not only didn't know Angular, they didn't even know much at all about web development in general. They were let go three days later. I'd heard of things like this happening but I couldn't believe it until I saw it in real life. I just can't imagine what someone like that is thinking. I get "fake it 'til you make it" but this was on a whole other level.
I wasn’t a part of the interview process, but I was told some of the details. The interview was remote and they did well on the video interview and on the take home challenge. My best guess is they were being given help.
Funnily enough I have a credit card sized (well, roughly triple thickness) thing in my wallet, never occurred to me before now, hasn't been flagged in X-ray yet. (I suppose now I'll remove it if I remember though.)