Just to add to this, this is not DM-controllable, and like a lot of elements of D&D Beyond, it's not consistent.
DMs have zero ability to control what options players see/have. If content is owned or shared, players see it. Even if you disable the sharing, it's still shared for character creation and so on (it just blocks them from reading the entire book), which is a little surprising though there is a note on Beyond carefully explaining it.
And the "show legacy content" toggle is in a weird place in character generation in Beyond. When you create a character initially, you get a giant page full of toggles, most of them controlling what content you see (this is for the player to pick, not the DM, surprisingly), like Eberron, Rick & Morty, Critical Role, etc. - and there are two Tasha's toggles, one for "customizing your origin", and the other for "optional class features", which default to off. One would expect "show legacy content" because literally everything else that controls content is - but in fact it's on the race choice screen, as a toggle there (and not entirely well-explained).
