evileeyore
Mrrrph
Previous to 3e there was no "core/not core" split, as Hussar explicitly said up thread.It must be a regional thing. But honestly, nobody I know would ever have assumed you could use something in unearthed arcana without DM approval. Everything in the core books was assumed unless the DM spoke up. That to me is what core means. Assumed or not assumed.
Everything printed by TSR for D&D was simply D&D. You either used the rules or not, no "These books are core and everything else is optional" since everything was optional.
Do you grasp that concept? The only rules used were what the DM wanted to use (and in many cases what the players and DM negotiated to use).
No, 4e simply went back to oD&D, BECMI, and AD&D standards, everything written by WotC for D&D is a rule for D&D to be used or not at the DM option (or as negotiated between players and DM).It might be that 5e is the first game where nothing is core as opposed to 4e which proclaimed everything core. Though I'll add that even then I didn't know many DMs that automatically allowed stuff beyond the first three books. Perhaps old reflexes die hard.
3e (because of the OGL) was the only edition of D&D to define "core" and "not core" and that was to avoid the nonsense of escalating splat books*.
* A term which didn't even exist until the mid '90's and arose from the intersection White Wolf's expansion book naming convention and the laziness of newsgroups posters.