In the past, when a station had programming and engineering people on staff who cared a great deal about their on-air sound, yes, they would often make extensive tweaks to their copies of older songs to assist the processing in producing the best on-air sound possible for each song. Those tweaks could consist of absolutely anything: linear equalization, de-essing, taming screechy upper mids or honking midranges by appying gentle compression to just one frequency range, light amounts of AGC, de-noising, or even just finding obscure out-of-print copies of songs that there were no decent-sounding modern issues of. Michael Hagerty mentioned this being one of former KRTH chief engineer Lynn Duke's passions when the station was still running 50s-70s oldies, for example (
see here).
I got to talk with Lynn a few times during the Usenet era, and can add that he told me KRTH had two identical airchains -- i.e., a clone of the famous fogged cabinet, made up of all the same boxes, each modified and adjusted in exactly the same ways. The reason was so the station could transparently switch to the auxiliary chain if anything happened to the primary. But knowing that, I wouldn't be surprised if Lynn used that auxiliary chain whenever tweaking songs in the station's library. With it just sitting around idle otherwise, it would have made perfect sense to utilize it that way, so as to be able to hear exactly how each tweak was interacting with the station's processing in real time. Absolute best way to achieve perfect results.
The industry being what it is now, I doubt much of this finessing still goes on. There's less need for it with 50s/60s oldies now as scarce as they are. The intelligence in modern digital processors is also much better at correcting disparate source material for consistency than it previously was.
And then you have the sound of eCBET and Voltair
enshittifying the sound signatures of most stations in PPM markets. That ought to be enough to demoralize any engineer out of spending time polishing much of anything these days.