That's Rath Games and he's a cinnamon roll of great justice.
But he placed arbitrary restrictions on himself that are mostly just for the challenge he wants.
He is purposely throwing away quest rewards. Had to make several allowances already. Needed streamer privilege to get enough folks to do Crystal Tower raid series. Even then, he opted into making it so that if he dies, then they wipe. At least until World of Darkness.
Notably, he doesn't actually ban tomestone gear, but does try to not use it, if it gives him too much iLvl for what he's doing. All in all, though, he's also refusing to use Epic Echo and other things instituted into the game that are there to account for the stat squish on older content. Unless absolutely necessary, whenever he deems it so.
I'm not trying to knock him. His streams and vids are phenomenal, and he is my second most favorite FFXIV content channel right behind Cider Spider (though admittedly, I'm more of a parasocial one way friendo in relation to Spider, cause Spider is just so damn cool).
As it stands, Crystal Tower is the only place you have to make a natural exception for, or go out of your way to gather other Ironmen or Ironmen-adjacent players for. Other than that, you can grind to your heart's content and slowly but surely get up to max level without leaving ARR. No idea how long that'd take with reduced Fate EXP, but you'd have the challenge log for weekly spurts.
I myself started a character where the purpose of the character was to try and jail break the dungeon order, and sure enough, the 5 side dungeons that don't require any MSQ completion are Sunken Temple of Qarn, Cutter's Cry, Dzemael Darkhold, Aurum Vale, and Wanderer's Palace.
It's actually fully possible to level up and beat Wanderer's Palace with Red Mage or Samurai with their i115 quest reward sets in the mid 50s, solo, without any good accessories, before even unlocking Sastasha. There were some veteran or CE reward feathers for buying tomestone i130 gear too, so you could get those as well. Rath makes use of those at some point to get himself Shire gear as I recall.
It's really not that hard of a challenge, unless you impose exactly what Rath does on himself.