To paraphrase JRA-7272:

One additonal improvement to conditions, permissions and customisations to workflows would be to allow combinations of transition conditions to use ... NOT logic.

JRA-7272 resolves AND and OR logic, but the resolver says, "File another New Feature request for NOT logic".

I can't find such another request, so filing this.

The ability to use NOT in regards to a condition would be useful, say, for something like "(User in group X) AND (NOT (User is reporter))" to make sure the reporter say can't approve the issue they requested.

Without NOT, have to write duplicate condition plugins to do the same operations as existing conditions, just to throw NOT logic in.