Protobuf is the example I had in mind. It uses sun.misc.Unsafe which is being removed in upcoming Java releases, but it has a slow fallback path. It logs a warning when it runs if it can tell it's only using the fallback path but the fast path is still available if the application owner set a flag to turn it back on if they want to:

https://github.com/protocolbuffers/protobuf/issues/20760

Java Protobuf also logs a warning now if you can tell you are using gencode old enough that it's covered by a DoS CVE. They actually did a release that broke compatability of the CVE covered gencode but restored it and print a warning in a newer release.