FWIW, Samsung used a plastic back for their basic Galaxy S21. I like it personally (less slippery than my old S7 Edge), but it seems to have been controversial among the user base, and they switched back to glass for later versions.
Thanks for the tip! I'll have to watch and see if it gets Lineage eventually as I don't really care for Samsung's software, but by the time I'm ready to replace my OnePlus it might be a contender.