Perhaps used to be a GOP district would be more appropriate...many districts in Ohio, Indiana etc used to be GOP.
No, we’re talking about ones that basically and historically vote GOP for President, statewide, downballot, legislative, etc. A Republican occupies the NOLA seat in LA, and we wouldn’t call it a “Republican” district. In IN, for example, there’s really only 2 out of 9 Democrat districts (and reconfigure the Indianapolis seat, and we can reduce it to 1, all but the Lake County-based 1st district).