This is remarkably on-brand for Games Workshop.
As I said when "Warhammer+" was announced, this would be a boon or a bane depending on how they dealt with fan animations going forward. This policy move is consistent with Games Workshop's policy towards its audience and customers since its transformation into a full-on corporation and the end of it being Shit Some Guys Thought Was Fun.
It is no surprise that SODAZ and others that made these popular fan animations have put out the word that they're going elsewhere.
It is not likely to stop fan animations from getting made, even high-quality ones, or being passed around. It's just going to be done deniably, much like SODAZ and Peterson can plausibly deny the reuploads of their works to YouTube off of their channels.
However, from GW's perspective, this is no different from recasting models or using 3D sculpts of the same and spreading PDFs or other copies of the rules around. In short, they see fan animations as others fucking with their money and thus see it as a threat to be exterminated and not an ally to be cultivated into a fruitful relationship. It is remarkably short-sighted and ultimately unviable, but that too is on-brand for GW.
Couple this with GW announcing the return of Warhammer Fantasy Battle under "The Old World"--complete with square bases--and the behavioral tells reveal that GW is a company on the back foot (again) trying to stop something they can't control (again) despite reality not being on their side (again) because they are constantly trying to head off their own audience to wrest control from them.
Games Workshop is going to learn what other companies learned the hard way: they can't.
Once the fans realize that there are webhosts, payment processors, and other online infrastructure beyond the reach of these companies and the state institutions they lean on to do their legbreaking for them watch for a massive preference cascade to towards those alternatives to hit for fan creators who just want to put their shit out for people to enjoy- especially if it's better than the official versions.
(Hint: Russian and Chinese options won't fuck with you so long as you aren't messing with their internal affairs.)
There is a superior policy, and Lucasfilm (under Uncle George) showed the way.
The official position should be disavowal of fan-created media, meaning that they are not recognized as official and nothing said or shown shall be reflected in official products. Only trademark gets aggressive enforcement (because the law requires it; it's Use Or Lose); copywrite enforcement should be more lax akin to a Don't Ask/Don't Tell affair, but covertly it is monitored for two things: emerging talent and gross violation.
The former is self-explanatory. The latter is primarily a matter of brand identity. The policy must include, at the least, a link to a statement that explains what the brand identity is in terms of relevant categorization with relevant examples--"Star Wars is a Space Opera adventure brand in the mode and tradition of E.E. Smith's Lensmen first and foremost, with influences from Flash Gordon, Buck Rogers, and rooted ultimately in the adventures of E.R. Burroughs' Mars and Venus series."--and enforcement of copywrite is primarily against off-brand creations. When possible, post a Series Bible for fan creators to follow that defines the brand.
The reason this sort of policy is reasonable is because it recognizes that IP cannot be locked down anymore. Limited time is used to clear the garden of weeds, not to rip out all but the very spergy flower choice you want, because the result of creation when it hits the public can be unexpected but welcome Black Swans rarely are actually wholly defiant about expectations held by the audience of what that brand is about.
But Games Workshop isn't run by reasonable people. It's run by the Mammon Mob, and they're now begging to get burned by their own customers avoiding giving them money.