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
2.) Bewear, Braviary
3.) Chandelure, Conkeldurr
4.) Darmanitan(All forms) , Decidueye, Delphox, Dodrio, Dragonite, Durant
5.) Electivire, Feraligatr, Flygon
6.) Garchomp, Gyarados, Golisopod
7.) Hariyama, Hawlucha, Haxorus, Hydreigon
8.) Infernape, Jolteon, Kommo-o
9.) Mamoswine, Metagross, Mienshao, Noivern
10.) Porygon-Z, Primarina, Pyroar
11.) Rhyperior, Salamence, Slaking
12.) Snorlax, Staraptor, Starmie, Stoutland
13.) Swampert, Talonflame, Tauros
14.) Tentacruel, Typhlosion, Tyrantrum
15.) Unfezant, Vikavolt, Volcarona
Bans For Gen 8: Inteleon, Rillaboom, Copperajah, Dragapult, Grimmsnarl, Barraskewda, Sirfetch'd, Cinderace, Kangaskhan, Lucario, Basculegion, Ursaluna, Hisu-Arcanine, Sneasler, Hisui-Goodra
Fresh Bans on Gen 9: Lokix, Quaquaval, Pawmot, Kilowattrel, Baxcalibur, Meowscarada, Skeledirge, Gholdengo, Archaludon, Annihilape, Revavroom.
Banned Moves : Venom Drench and Explosion/Self-Destruct Banned. Recovery Moves Banned. 151 BP Ban.
Some Noteworthy Usable Mons: Mega Audino, Mega Altaria, Mega Sableye, Mega Medicham, Mega Mawile, Brute Bonnet, 10% Zygarde, Fezandipiti.
Updates : Revavroom Banned.