Personally I use the thread made by Perusoe when it comes to finding D3 patch notes.
As for patches 2.6.2 and 2.6.3, I sadly can’t remember anything about 2.6.2, but I can say that 2.6.3 was for the switch release, as that was released around November 2nd, 2018 (iirc). As for what update was for season 15, I can’t say for sure, as I don’t believe we had an update for that (not a major one anyway), since at that time we were going through a drought when it came to updates (in short we were going a long time without an update). So season 14 and 15, both of which whose themes were active in both non-season and season as they were double goblins and double bounties respectively, were after update 2.6.1, but before update 2.6.3 (switch release).