Is Tasha's Cauldron of Everything, considered a "legacy" book now?  We're using 2024 core rules and a player wants to get a magic tattoo.  It doesn't seem clear if they are legacy items.

I'd say they are still valid, since magic tattoos aren't replaced by anything in the new core rulebooks.

Apart from some external articles and interviews with the Dev, this is the official information from Updates in the Player’s Handbook (2024) | Dungeons & Dragons:

When you play D&D with the 2024 Player’s Handbook, it replaces all rules, classes, subclasses, spells, feats, equipment, species, and backgrounds in the 2014 version of the book. There are a few exceptions; the following options don’t appear in the 2024 book and are still usable from 2014: [...]

Any subclass, spell, or feat from Xanathar’s Guide to Everything or Tasha’s Cauldron of Everything that appears in this book replaces the version that appears in the Everything book.

The only thing that's universally not applicable to 2024 content from Tasha's in the additional class features- at lot of them got integrated as core options in '24, and the rest presumably just didn't make the cut. For pretty much everything else, unless there's been a reprint in '24 content, it's still the most current model and thus is the default option for current state of play.

 Thanks to you both.