After Ubuntu Studio upgraded the Audacity package to 2.4.1, many effect plugins were missing (for example, plug-ins/highpass.ny -- several ny files from this folder are missing).
I asked on the Audacity forum and they said, no, these effects haven't been removed and that it's a packaging issue. They advised me to contact the package maintainers, which I'm doing now.
So I'm now building from source by myself.
Just reporting, I hope this can be fixed in due time.
hjh