Hello,
well, sadly you are not the only one feeling concern about the PHP8 “”“upcoming”"" …
So far as I could understand, PHP8 is not on the roadmap for ownCloud 10, Infinite Scale won’t replace ownCloud 10, but there will be a migration path to migrate from ownCloud 10 to Infinite Scale …
If it makes any sense to you 
Some ressources about this whole endless no answer topic:
Cheers !