Thanks a lot! I found an updated version of phpQuery that works with PHP 8 and replaced the older version within the kirby-anchor-headings plugin. For now it works again. Seems like i have to make the step to Kirby 3, but maybe I can manage to wait until Kirby 4 to make the transition.
Kirby 2 reached EOL on January 1, 2021, i.e. more than 2 years ago. As I already mentioned above: Kirby 2 just does not support PHP 8, you would have to make changes to the source code to make it work, because PHP 8 came with a lot of breaking changes. And with an installation that has that many plugins installed, it will be even harder.
I’d recommend you pay your hosting service the excess fee they charge for prolonged PHP 7.4 support, if you cannot update to a newer version of Kirby now. Or if that’s not possible with your current hosting, then find another one, or set up your own server.