Nah, Redis users would just keep using the last free version. Probably many of them will do that anyway because they missed the memo about Valkey. Like the poor people still using OpenOffice.
I guess this is more okay when the dated versions are not exposed publicly but are backing services, but in general it might be a question of time until some CVE puts you at risk or there's a bug that will never be fixed.
I like the idea of having "feature complete" software that you can use from now until the end of time (or at least the end of whatever product you're working on), I just don't think this is always feasible with the way we develop software.