There is no blue post or interview to support this claim. In fact, in counter to this claim it has been pointed out that right now farming in retail is easier as the raid lockouts will return to weekly instead of daily for farming after remix ends.
In response to an earlier comment about quests not dropping the items they do in retail, you’ll find on some quests that finishing the quest gives you some appearances.
I still want to know specifically if you can get the pets from fishing. I have spent multiple hours fishing with no fish pets dropping, but got one very quickly on regular retail, so I would really like to be 100% sure on whether I’m suffering pointlessly on a timerunner.
The only mounts that historically have a higher drop rate when the content is current are from final bosses in raids. In MoP’s case, this would only apply to mythic Garrosh.
Elegon, Ji-Kun, the world bosses, and the warbringer mounts did not have higher drop rates back when MoP was current/did not get reduced at any point since then. In fact, the world bosses actually had lower drop rates back then than they currently do.
If you have examples of mount drop rates that were lowered after an expansion ended besides mythic raid mounts, I personally can’t recall any so I’d be interested to know!