-Ysafe-init is not documented in New Compiler Options, nor is -language:strictEquality.
That page has the disclaimer “The current page only contains the options that were added in Scala 3.0.x.” (which should probably be addressed). However, I didn’t see these flags mentioned in the release notes for Scala 3.1, so I guess they were already available for 3.0.x (for some x).