The deprecation warning for Harness says:

Harness is deprecated; we recommend using state transition testing (previously known as 'Scenario') instead

We should do a better job helping the charmer move forward from this. Just pointing to the docs is potentially enough (I don't think they were live when we added the warning). Maybe we also want to say to install ops[testing], that the classes are still in ops.testing, and things like that as well.

Pointed out by @jameinel in Matrix.