Since this seems to have the most up-to-date information regarding Steam, I don't think it'd be a bad idea to post here. One alternative I can think of is making Steam launch a launcher that allows you to select a build or asks if you want to update with a disclaimer.
Optimally I'd just prefer you release it into Early Access with a disclaimer saying their saves will probably break, making an announcement explaining how they can prevent that (aka disabling updates) in the Steam Discussions, and ignoring incompetent people that fail to read and understand Early Access.
Optimally I'd just prefer you release it into Early Access with a disclaimer saying their saves will probably break, making an announcement explaining how they can prevent that (aka disabling updates) in the Steam Discussions, and ignoring incompetent people that fail to read and understand Early Access.