Last year, I pointed out how many big publishers came crawlin' back to Steam after trying their own things: EA, Activision, Microsoft. This year, for the first time ever, two Blizzard games released on Steam: Overwatch and Diablo 4.
No exception was provided for this last time I looked through the subscriber agreement. I’m not real keen on reading through it again. Can you share the relevant portion?
As for the big red button at Valve, I can only find snippets of information. The only "official" one: https://imgur.com/4sa1Ln6 Most information is just from interviews or private messages, claiming there is an inner senior circle at Valve and they do have a contingency plan.