Using LLVM's legacy PM for optimization pipeline was deprecated in 13.0.0 and the relevant functionality was scheduled to be removed after 14.0.0 (got delayed). The legacy PM for code generation (assembly, object file generation; mainly addPassesToEmitFile) is still supported.

Clang dropped legacy PM support in D123609 (milestone: 15.0.0). I plan to remove instrumentation passes like gcov, PGO, and sanitizers (milestone: 15.0.0).

LLVM's legacy PM for optimization pipeline may be completely removed in 16.0.0 (llvm::create*Pass, populateModulePassManager, etc). llvm-project clang/lib/CodeGen/BackendUtil.cpp can serve as an example using the new pass manager.