We recently provided some information about which version of SRP is compatible with which Unity Editor version, and how we will update/support these in the future.
TL;DR:
SRP 8.x is the supported version for 20.1.
SRP 9.x is a preview version for 20.1. It will never leave preview; it is intended as a way to test out upcoming 10.x functionality in 20.1.
SRP 10.x will be the supported version for 20.2
More details:
Reminder: Always keep your SRP package versions the same for a given project!