RU is known as The Rarely Used tier. This tier is not made in a way that's competitive but instead aims to be fun which lets low or similar power level Pokemon to be usable, even some usable Legendaries and Megas, which are all mentioned.
Banned Pokemon :
-# Ordered alphabetically
1.) Aerodactyl, Alakazam, Arcanine, Archeops, Annihilape, Archaludon
2.) Barraskewda, Basculegion, Baxcalibur, Bewear, Braviary
3.) Cinderace, Chandelure, Conkeldurr, Copperajah
4.) Darmanitan(All forms), Delphox, Dodrio
5.) Dragapult, Dragonite, Durant, Electivire, Falinks, Feraligatr, Flygon
6.) Garchomp, Gallade, Gholdengo, Glimmora, Grimmsnarl, Golisopod, Gyarados
7.) Hariyama, Hawlucha, Haxorus, Hydreigon
8.) Infernape, Inteleon, Jolteon, Kangaskhan, Kilowattrel, Krookodile, Kommo-o
9.) Lucario, Meowscarada, Mamoswine, Metagross, Mienshao, Magmortar, Magnezone, Noivern
10.) Pawmot, Pinsir, Porygon-Z(Event too), Primarina, Pyroar(Event too), Quaquaval
11.) Revavroom, Rhyperior, Rillaboom, Salamence, Sinistcha Sirfetch'd, Skeledirge
12.) Slaking, Snorlax, Sneasler, Staraptor
13.) Starmie, Stoutland, Swampert, Talonflame
14.) Tauros, Typhlosion, Tyrantrum
15.) Unfezant, Ursaluna(All forms), Volcarona, Wishiwashi
Banned Moves : Venom Drench, Explosion, Self-Destruct Banned. Pure Recovery Moves Banned. 151+ BP Ban. Breeding Moves.
Noteworthy Usable Mons: Includes Regionals, Legendaries, Megas and Mythicals. Check ?tag ru-les
Updates
Special Rules and Noteworthy mons shifted to ?tag ru-les.
Banned - Rillaboom, Porygon-X, Gallade, Wishiwashi, Magmortar, Brute Bonnet
Unbanned - Mudsdale.