The Forgotten Realms shares a multiverse with Oerth and Krynn (and Earth—see several of the gods and some cultures like Mulhorand). Just because alternate FR haven't been explicitly said to exist, given that worlds that the Realms connects to do and somehow the Realms is an exception.
But the Forgotten Realms is part of the D&D multiverse.
There are not alternate versions of the Realms.
The default assumption of D&D since the earliest days is that of a branching multiverse like in classic comics
Not for The Forgotten Realms.
I see no reason to assume that, the Grey Box itself states DMs are expected to make it their own and make any changes they like, which seems precisely in like with a standard D&D branching multiverse assumption. I have seen no statement, anywhere, that would contradict that inference, which has been canon explicitly for years now at any rate.
So no actual evidence.
Sure, plenty, from OD&D to the 1E PHB and DMG (both of which use the word multiverse explicitly in the branching timeliness version), and the Grey Box. Seems well established and older than I am, even.
That doesn't apply to the Forgotten Realms having a multiverse.
No evidence against it exists either, but more than that, it is explicitly canon at this point, though it can be easily inferred from 80s canon.
