Just a little insight: I was interested in using my spare unused 2Pacs for some solomining. It was working with 0.19.1 but it broke after updating to 0.20.0 and I was looking at the code and figured out that this pull request was causing the problem[1]. If you want to mine with 0.21.0/0.20.X instead, then you'll probably have to either modify your mining client somehow to ignore that missing part in the scriptsig and compile it.

SoloMining with 0.19.1 is perfectly fine, if you don't want to complicate things.


[1] https://github.com/bitcoin/bitcoin/pull/17519