Update v2: Gen 8 Pure Hackmons is now playable on Showdown.
A "new definition" of Pure Hackmons sparked the process of the deletion of BDSP Hackmons and the revival of Eternamax.
From now on, anything directly hackable onto a set (EVs, IVs, forme, ability, item, and move) and is usable in local battles are allowed. This new premise redirects the focus of Pure/Balanced Hackmons to the Pokémon themselves; meaning that we're only taking all Pokémon already programmed into the respective game a Pure/Balanced Hackmons format is based on and allowing anything that can be hacked onto their set.
Source
Sword and Shield Pure Hackmons will now be available to play on PS! through roomtours and challenges. You can get battles in the Old Shark room.
The game would still work around the flirtatious 30% accuracy OHKO moves, though, as Overflow still exists, as it isn't a hack. Eternamax can't be banned, too, as it's programmed as a regular Pokemon forme having its own set of base stats and moves, unlike the Dynamax/Gigintamax formes. There still isn't any homogeneous winning strategy, the metagame is back to almost what it was pre-BDSP.
SS replaced BDSP as the host for Generation 8 Hackmons because BDSP isn't faring any better. It's a single-Pokémon metagame and not very interesting or open for newer users.
Another reason they gave for the comeback was way deeper and more coherent:
Additionally, since the video on Pure Hackmons made by freezai, there has been an upward spiral of demand for this format to be playable again.