Greetings to all the latecomers of Shadowlands. I had been trying to find out how to unlock mounts in the Maw, and I found lots of guides online, but I was getting frustrated because every time that a guide told me to go find a specific quest - ya, that quest was not available to me, and the guides did not explain why. The guides that were all written around 9.1 when this content dropped fresh, and the people who wrote the guides had already progressed their Campaign/Covenant/Renown far enough before 9.1 dropped that they couldn't really pinpoint what was the exact prerequisite that unlocked these quests. I figured it out, so here's this post to help others with the same question.
I unlocked mounts in the Maw during patch 9.2.5 in July 2022. I was only at Renown level 18 with my Covenant, so you don't need a high renown for this. The prerequisite to the questline was very simple - just keep progressing the main Campaign/Covenant/Maw quests that you get after you hit level 60. Here are the notable quests I completed in the lead-up to unlocking mounts in the Maw. I did my Covenant/Maw quests until they directed me to Torghast, then I met the Runecarver, then I did a couple quests to assist the Runecarver. After I helped the Runecarver to break something, I was directed to Oribos, then I was directed to speak with Bolvar Fordragon, and that conversation led me to a story driven questline - "The Battle of Ardenweald". After completing that questline, I accepted a quest from Tal'Inara in Oribos, where she inspected my anima, and that is the beginning of the very brief questline to unlock Korthia within the Maw, and during that questline you will unlock mounts. That's it. Just keep doing your Covenant/Campaign/Maw quests, and you will be directed to Korthia and get your mount access in the process.