This is why third party CMS themes and plugins can be a minefield. You're dependent on code you didn't write/don't know about to keep your site working, with no guarantee it'll be updated/keep working in future (especially if said theme or plugin is by a company that no longer exists, author that doesn't work with that software anymore, etc).

Sadly there aren't many good options at this point. You can try and fix the offending plugins yourself, though that can be far more difficult than even rebuilding them yourself. You can try and rebuild said plugins from scratch, but that's also a lot of work, especially if you have to keep some form of backwards compatibility. Or you can remove the features in question, which may not be practical if the plugin is deeply integrated into the structure of the sites.

Out of curiosity, what sort of things do these plugins do though? Maybe we can figure out if other solutions exist to replace them, or whether these particular features are still needed in 2023.