McCoy was in Encounter at Farpoint with one meta purpose - to counter the TOS fans that were campaigning hard to say that it ‘wasn’t the same universe.’
McCoy’s presence was a nice Easter Egg, but not much more. But he did the job of saying that it was the new Enterprise in continuity with the legendary ship on which he served.
Fans argued that because Roddenberry insisted on moving WW3 back to the mid 21st century as of Encounter at Farpoint, TNG had to be a different timeline.
TOS fans understood the Eugenics Wars to be the precursor to WW3, so they just didn’t accept WW3 was going to be another half-century away. Roddenberry’s directive was to always keep the Star Trek future in our future so WW3 had to be shifted to later in time and any specific mention of the date of the Eugenics Wars was avoided.
They also hated the carpet and many other things about the ‘luxury hotel in space’ Enterprise.
Yup, that happened and continued to happen until well into TNG season 3. The brigading Berman-era fans who rail unrelentingly against ‘Nu-Trek’ don’t sound any different, they’re just more visible than the 1980s fans that relied on mimeoed fanzines and Usenet. Fans that liked TNG kept quiet at cons until at least 1990, and vendors didn’t bring TNG merchandise.
Roddenberry himself was adamant that Star Trek’s history had to remain a possible history for viewers. So, the dates can slip as long as the major events don’t.
That is why he put WW3 later than implied by TOS, delaying it to the mid 21st century in the TNG pilot ‘Encounter at Farpoint’ even though that led to a contingent of TOS fans insisting that it ‘had to be a separate universe from the one of the original series.’
While writers never explicitly resolved this onscreen during the Berman Era shows, preferring to weasel with offscreen head canon in interviews saying that perhaps the Eugenics Wars were covert and going on unknown in the 90s, the new shows have dealt with this problem head on by acknowledging that temporal incursions do affect the timing of major events without making it a separate timeline.
SNW and Prodigy have been able to make this clear onscreen in canon with the expert help of the franchise’s excellent physicist science advisor Dr. Erin Macdonald. (She did her PhD with the team in Scotland that got the Noble prize just a couple of years later. She’s truly on top of modern theoretical physics.)