Depending on what you mean by "in some future update" and "pretty much guaranteed" (given an infinite timespan everything will disappear) I don't think that's true. I've kept my Firefox user.js (my manual about:config changes) under git over the past 3 years[0], and of the 44 options that I customised, 36 are still present and (seem[1] to be) active.

6 of the about:config user_prefs customised add-ons, so they no longer work due to the shift to Webextensions (but I can still make 5 of the customisations via another interface).

1 customised the GCLI, which was removed, and 1 customised Panorama, which was also removed. (However, most of what GCLI did, can be done some other way, and there are a couple of Webextensions faithfully emulating Panorama.)

[0] The file is older, but I added it to git only three years ago. Hence, many of the about:config changes have been "alive" for longer, but I have no record of those that were removed earlier than 3 years ago.

[1] Cursorily glancing through my comments above each entry to make sure that it still does what it was supposed to.