I'm also in the PHB + Xanathar + Tasha's camp, but far less likely to accept something outside of it. Also some subclasses from Tasha's I wouldn't allow.“PHB+3” and “anything” are both technically accurate for me. I say you can use PHB, Tasha’s and Xanathar’s freely, and if you want to anything else, talk to me and I’ll work with you to figure out how to make it work in the campaign.
The key with stuff outside those books is we’ll probably have to re-work it. Like, for my upcoming game I have a player who wanted to play a Mark of Shadow elf. Now, Dragonmarks are an Eberron-specific thing and this isn’t an Eberron campaign, so obviously they can’t actually be a Mark of Shadow elf. But, the mechanics are basically just elves with some illusion magic. No reason they can’t use them, we just reworked it as a Dusk Elf from Ravenloft.I'm also in the PHB + Xanathar + Tasha's camp, but far less likely to accept something outside of it. Also some subclasses from Tasha's I wouldn't allow.