There is some Russian website stealing litteraly thousands of mods and releasing them as a pack. Without the consent of the mod authors.

That "mod" is called Skyrim: The Journey.

Is Bethesda going to do anything about it?

I messaged a few people about a few specific mods that were pointed out to me today on a Russian site. You should let people at Bethesda know. I'm not sure how they handle it (if at all) or who within does so.

The first thing you should do, even before making this thread, is contact all the modders that are included in the pack and let them know.

Mods have been getting hosted where they shouldn't for the last 13 years. And so far Bethesda has never had to intervene (as far as I know.)
The community is really good at taking care of thieves, because we rally together to get them shut down.

So your top priority should be to inform the modders who are having their mods used erroneously.