The zone of https://vanillawowdb.com/?zone=361 Felwood has a series of repeatable quests that begin with the quests https://vanillawowdb.com/?quest=4101 Cleansing Felwood and at least one of the follow up repeatable turn in quests (for the salve):
https://vanillawowdb.com/?quest=4106 Salve via Skinning, https://vanillawowdb.com/?quest=4107 Salve via Disenchanting, https://vanillawowdb.com/?quest=4108 Salve via hunting, https://vanillawowdb.com/?quest=4109 Salve via Mining, and https://vanillawowdb.com/?quest=4110 Salve via Gathering.

Once you have some https://vanillawowdb.com/?item=11516 Cenarion Plant Salve you can do the quests:
https://vanillawowdb.com/?quest=2523 Corrupted Songflower which leads to https://vanillawowdb.com/?object=164882 Cleansed Songflower which gives https://vanillawowdb.com/?spell=15366 Songflower Serenade
https://vanillawowdb.com/?quest=4119 Corrupted Night Dragon which leads to https://vanillawowdb.com/?object=164881 Cleansed Night Dragon which gives https://vanillawowdb.com/?item=11952#c0z Night Dragon's Breath
https://vanillawowdb.com/?quest=4117 Corrupted Whipper Root which leads to https://vanillawowdb.com/?object=164883 Cleansed Whipper Root which gives you https://vanillawowdb.com/?item=11951 Whipper Root Tuber
https://vanillawowdb.com/?quest=998 Corrupted WIndblossom which leads to https://vanillawowdb.com/?object=164884 Cleansed Windblossom which gives https://vanillawowdb.com/?item=11950 Windblossom Berries

It is worth mentioning that when cleansed the plants will stay that way for something like 2 hours. Their quests can only be completed like every 10-15 minutes though.
This means you can get these buffs/items pretty consistently for 16 levels (the starter quest can be accepted at level 48, and the buffs continue to give the buff through 63) as long as you aren't dying very frequently and you actually do enough quests in felwood to keep your cenarion salve in stock (don't throw away any corrupted soul shards)

A lot of other Vanilla unique buffs seem to be unavailable after 63 as well ( https://vanillawowdb.com/?spell=22817 Fengus' Ferocity , https://vanillawowdb.com/?spell=22818 Mol'dar's Moxie , https://vanillawowdb.com/?spell=22888 Rallying Cry of the Dragonslayer, https://vanillawowdb.com/?spell=22820 Slip'kik's Savvy, and https://vanillawowdb.com/?spell=24425 Spirit of Zandalar), seems that's when they want you to give up on azeroth buffs.

This quest is no longer in game as far as I can see.