So I upgraded from 1.22.x to 1.23.5, which completely broke my entire wiki. The new skins functionality first warned about errors in skins, just as described in the upgrade log. I then attempted to correct this by adding the require_once text described in the manual skin talk page to the LocalSettings.php, which produced an endless long list of new errors I have no clue about. So I then tried to delete unnecessary skins except the one I use, no change there.
Then I tried excluding the part I didn't understand about pointing to the non-existing MySkin.php file, and instead in LocalSettings pointed to my own skins theme file. This didn't work, since I then got a ton of errors from the skin file and I have no idea how to modify this file to work.
So lastly I tried changing LocalSettings to point to Standard.php skin file, this still didn't work, so now the whole site is down, and I'll be trying to do a restore instead. That is of course a nice excercise but this while thing just once more reminds me that what I really should be doing instead of upgrading is migrating to WordPress.
I even tried to redownload the full 1.23.5 to extract just the MySkin.php file to use that, but it wasn't there so now I have completely lost myself in a maze of errors.
If only now I can get back to the previous version I believe I'll leave it there until I can migrate content. Security issues or not, upgrading seems to effectively kill the site which to me is worse than what any breach can achieve. Unfortunately I see no other way out of this right now.