Not just that, but also 4in6 (IPv4 in modern networks) and 4over6 (an extension to DS-Lite), and don't forget 6in4 being called SIT in an early draft, followed by SIIT being used in another protocol with a similar purpose (though that's more of a "Linux kept the old name around while everyone else moved on" problem).

Like most actually-used transition mechanisms, 6rd the result of someone inventing a transition strategy for a very specific use case, a company with that use case actually using that solution, and followed by standardization for good measure.

6to4 has been disabled for a few in Windows and has been disabled by default in Windows 11 since release, so in practice you probably won't see it outside of legacy networks and the routing systems designed to support 6rd or legacy networks.

Teredo was a weird Microsoft thing (like so many other network protocols) that could've replaced stupid shit like STUN and TURN had it been used. I'm pretty sure only Microsoft ever bothered to host Teredo servers to the mainstream. It would've been nice to live in a world where STUN and TURN worked for any protocol without having to set up your own servers rather than just supporting UDP and the server of an ad company (like we use today).

6in4 was a naive attempt at making the transition to IPv6 possible in the very early days (1996). It's the response to all of the "what if we just added a bunch of bits to IPv4" suggestion, and it turns out that nobody who says that actually seriously cares about supporting IPv6.