Is this a new lore thing? Wookiepedia just gives the old EU lore where sentinels are basically just "Jedi Knight- but trained to solve problems without using lightsabers or the Force". From what I can recall the crystals were just part of how KOTOR 1 distinguished them as seperate from "Stabby Jedi" (Guardian, blue blade) and "Magic Jedi" (Consular, green blade). There's a mention of Sentinels being relatively rare and a specialised form of them that served as temple guards but I can't really see where this "Sentinel=Super Special Elite Jedi" idea comes from.I know I'm ranting but I just have to point this out. Rey's Yellow Lightsaber means that she is a Jedi Sentinel. The Jedi Sentinels were the Guardians of the Jedi Temples and they were the Strongest of all the Jedi's. Their Lightsabers usually were either Yellow or Orange colored to show that they had mastered the force. There were usually about anywhere of 20-30 Jedi Sentinels, (I think,) to guard the Temples. My only problem with Rey having a Yellow Lightsaber, is that we don't see her getting the crystal. And from the look of the ending, she went straight towards Tatooine immediately after the defeat of the Final Order. Unless it took months for her to get to Tatooine, I don't know how she should have crossed a yellow crystal.
It's still pretty stupid that Rey should just get a free lightsaber crystal out of nowhere however. I'm sure Disney will eventually get around to explaining in a comic or book no-one will read how it's actually a missing lightsaber crystal of Ashoka's/Yellow Yoda's or something equally pointless.
On topic: Lol, rip in piss Star Wars.