Okay, got it. It *actually* was old code from different plugins. Some custom ones, some from the vendor directory, etc.

Once I cleared all out plugins that weren't compatible with 6, the cli ran again and I could do the upgrade.

A very weird error message though. But I guess you can't really optimize here when you've removed code which is called by legacy plugins.

Maybe the experience would've been better if I migrated from major version to major version. Maybe you could update the upgrade guide though because that is still stuck at upgrading to version 3.

Glad to hear this!

But...

Firstly, it's written in the manual.

Secondly, old plugins always need to be updated, as they are incompatible with previous versions of Elgg. And this is also written in the manual.

Thirdly, how to upgrade has already been discussed many times here 1, 2, 3 etc

Yeah, sure. It wasn't meant as a criticism. I know that I'm doing a loooong jump here.

I just meant, that the upgrading guide you linked only mentions versions up to Elgg 3. And apparently it's not just "update version in composer and run composer update". Especially when you have a lot of plugins installed. I guess it would be better to do it per major version as then the plugins that fail to support the new version could be identified better with less misleading error messages.