I don’t think so. On debian trixie it works perfectly fine for me. And it worked on debian 12 as well judging by daes comment in your linked topic (see Unicode alt code input on Linux not working with new launcher - #4 by dae).
It was automatically closed because there was no activity on it.
Can you use Ctrl+Shift+u outside of Anki?You already confirmed this.- Can you try the results that you get on the internet for searches like “linux mint control shift u not working”? E.g. this one: https://forums.linuxmint.com/viewtopic.php?t=320998
- Have you changed your keyboard related settings in any way (maybe indirectly by updating your distro)?
- Does it not work anywhere in Anki or can you find a window where it works?