Yaarel
🇮🇱 🇺🇦 He-Mage
I've certainly done more than my share of worldbuilding in D&D - I'm not much for published settings....
1e was what it was - and making it into something else was not easy, but extremely popular, back in the day. 3e was a PitA to work with, both because it was so detailed it even insisted on using the same mechanics for both PC and Monsters, and because of the communities RaW obsession, which made introducing house rules like pulling teeth. Yet I did extensive world-building for both of 'em. I re-used one of those worlds in 4e without issue, too.
5e is DM carte-blanche, the non-PCs of whatever sort can just be designed to the level of detail you care for, there's nothing stopping you from designing a world any way you care to, because you can change/introduce whatever you like.
The biggest problem in 5e, is the Players Handbook.
The world is spelled out everywhere in it. If the DM is trying to weave the illusion of a different world, it is impossible because the players are reading the Players Handbook.
For example, if all of the references to polytheism were in the DMs Guide rather than in the Players Handbook, it would be easier for the DM to use the core rules to build something that differs from Forgotten Realms.
Players Handbook even goes into detail about reallife polytheistic traditions that are superfluous and probably wont happen in most FR campaigns. But the polytheism of 5e is religious extremism.