Check out this Ashley's comment:
https://www.construct.net/en/forum/construct-3/general-discussion-7/games-nwjs-183964#forumPost1114476
He's talking about Local Storage, but I imagine system saves work the same way.
You can save the full state of the game to a file.
In that case, I'll need to stick with NW.js for this release. Do you happen to know if the Steamworks plugin as compatible with NW.js?
Edit - It does not support NW.js
For crying out loud... Well, changing all those achievements over was a waste of time...