A normal post, like this right now what I'm typing?

I guess it can be possible, but don't think that'd fly with relays in general I'd imagine... I think.

However, if you're talking about what's being posted in the current system of forums on nostr, a simple solution to that is to also have that system running as well (if people choose to use clients that use that system, for some weird reason, they can and it'd appear in the future system, since it's not necessarily a bad thing, but it's bad for that user using it and them choosing to use it). So ya, old posts wouldn't necessarily be left out =)