Pretty much exactly what the title says. Steam's phasing out the last (relevant) version of macOS, what will be done for folks who bought via Steam, if anything will be done at all?
#Steam support for platforms like macOS 10.14 going forward?
cyan rune
sweet pebble
Anyone who bought the game through steam has the ability to get it drm-free on factorio.com
so assuming the mac version there still works, that'll still be how its done