Reddit shadowbanned me multiple times for either absurd or political reasons, but my ISP used to assign dynamic IPv4 addresses, so I could easily create a new account. Then it switched to IPv6 with a static address, and now I can't create new Reddit accounts anymore.

Most consumer ISPs regularly rotate the prefix. IPv6 has proper support for that and can keep connections on the old and new prefix alive for a while in parallel.

With IPv4 most ISPs only rotate IPs on new connections (router restart). But my old home internet even stayed on the same ipv4 for years. Only if I shut down the router for a longer time it got a fresh ipv4 assigned.

So it's an implementation detail, not a technical limitation. Not rotating the prefix is a bad implementation.

Hopefully you mean a static prefix, a static address would be an abomination from an ISP in IPv6!