Riot Games seem to have been pushed into officially revealing Rell, the Iron Maiden, slightly early due to a leak that unveiled her abilities in a separate video. During the reveal, they also noted when we can expect her on live servers.
League of Legends' PBE usually holds new content for two weeks but an odd exception happens from time to time, usually when big changes happen - like the item overhaul. New champions are not considered as big of a change as the total item overhaul we're having on live right now and therefore, Rell is supposed to come out with patch 10.25, scheduled for release on Wednesday, December 9, 2020.
AD carries are currently having issues with agency and even being valuable enough to go with the entire risk of hoping to snowball enough for the endgame fantasy.
When Rell comes out, she will either make or break the entire lane since it's usually the supports who carry their marksmen until they become relevant. Her kit currently looks overloaded, allowing the teenage mage support to become extremely tanky, chain crowd control on enemies and heal allies while also buffing their armour and magic resistances.
This makes her perfectly suited for keeping the squishy marksmen alive against assassins who seem to be dominating the meta currently as they are able to hop in and out of stealth, able to instantly pop a squishy each time.
Naturally, it remains to be seen how much of the sandbox Riot Games can balance out before Season 11 begins but Rell is poised to become one of the most powerful supports in League of Legends before the holiday season.