Found when implementing unsafe evolution, mainly in #82043. The following list references test methods in UnsafeEvolutionTests class (it should be enough to replace unsafe with [Obsolete] to observe the bug; probably not everything is an unmanaged bug).

• Spreads (Member_GetEnumerator_Spread)
• Confirm whether obsolete DisposeAsync should be reported #30257
• Obsolete constructor doesn't produce warning when used to satisfy a constructor constraint #19430 (for unsafe implemented in Unsafe evolution: handle new() constraint #82647)
• well-known and special types and members
  • Lock object (Member_LockObject)
  • ITuple (Member_ITuple)
  • IDisposable (Member_Dispose_Interface)
  • all other
• Additionally, audit callsites of ReportUseSite, AssertNotUnsafeMemberAccess (e.g., types cannot be marked caller-unsafe, but still could have missing Obsolete checks), ReportDiagnosticsIfUnsafeMemberAccess.

Fixing this would be a breaking change.