No idea. Most recent example of a deleted account I recall is gameon, but there seem to be at least 3 gameon accounts, only one of which has disappeared. Searching for his posts will find some with one avatar, others with another, and some that only exist as quotes.
So no idea what happens with the accounts and posts.

Alternatively, it is possible that at some point GOG allowed users to post without an account.

Silly them ;-)

There used to be anonymous posting possible back in the days when Good Old Games were not so old as they are now. They disabled it over time and only registered accounts can post now but I guess they never bothered to remove the checkbox.

Ah, that would make sense, and would explain why I've never seen any - I very seldom go digging. Not sure why anyone would want to hide those, though. And how in the world did you find those posts in the first place??

As noted by Vitek in the interim, and in light of this post - upon examining the dates of the other posts in its thread - it's clear that the choice to post anonymously was an option sometime during the beta phase, and if deleted / disabled accounts are one source of those posts, that is the other.