Will ES2 be available on platforms other than Steam, not part of the anti-Steam circlejerk, but I would prefer to buy from GOG or anything but Steam. An amplitude based client would be okay too. Steam is known for not really caring for it's customers; with the recent caching error and the insanely slow response time for such a big company it seems quite ridiculous. For that matter will any other games be available on other platforms in the future. (Still gonna buy ES2 no matter what 'cus it's gonna be epic and we all know that)

Barring a response from the devs, I think the best response you'll get is "Likely not". Amplitude has always released on Steam. Even with the Humble Bundle and direct purchases, they linked back to Steam keys.

AFAIK, the games multiplayer relies on some of Steams API to work, so you have to have it with Steam.