The "Steam Open World Sale" is underway. If you can roam freely in it, be it an exploration game, builder, sandbox, survival, etc., odds are good that it's on sale.

There are some good ones in here, including a few freshly-prepared suggestions:

I'm not sure if there is a direct connection - RTS games are not the most suitable for an open world sale - but with the start of the new sale, the WW2 RTS Company of Heroes 2 and the standalone expansion Ardennes Assault will be given away. Ardennes Assault features a non-linear campaign on a dynamic map, which may be a bit of a stretch for an open-world tie-in, but it's free, which is good enough for me.

The Steam Open World Sale ends May 31 at 10 a.m. PT. This sale ends May 31 at 10 a.m. PT.