Am I insane or can't a company just fork it from before the license change? I mean, what even needs to change in it? I assume 95% of people were just using it for the features it's had since the beginning anyway.
The question - just like always in cases like this - is which forks will get long-term support. So just like with Terraform, it's probably a good option to stay on the last open source Redis version and wait to see how things shake out, assuming that there are no critical security vulnerabilities in that version of Redis. Alternatively, be prepared to jump around between a few forks if one turns into a dead end. Or move to something else altogether, but that's a much bigger undertaking.