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 below.
Banned Pokemon (Ordered By Alphabets) :
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, Feraligatr, Flygon
6.) Garchomp, 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, Noivern
10.) Pawmot, Porygon-Z, Primarina, Pyroar, Quaquaval
11.) Revavroom, Rhyperior, Salamence, Sirfetch'd, Skeledirge
12.) Slaking, Snorlax, Sneasler, Staraptor
13.) Starmie, Stoutland, Swampert, Talonflame
14.) Tauros, Typhlosion, Tyrantrum
15.) Unfezant, Ursaluna(All forms), Volcarona
Banned Moves : Venom Drench and Explosion/Self-Destruct Banned. No Retreat Banned. Recovery Moves Banned. 151+BP Ban. Mons with Breeding Moves.
Noteworthy Usable Mons: Mega Audino, Mega Sableye, Mega Medicham, Mega Mawile, Brute Bonnet, 10% Zygarde, Fezandipiti, Hisuian Zoroark, Hisuian Electrode, Hisuian Avalugg, Hisuian Decidueye, Overqwil.
Special Pokemons which are not mentioned in this list are considered Banned
Updates : Revamped Ban List. Check ?tag ru-changelog