So far 2019 is pretty meager with Linux and Mac additions, so let's take a look at some potential future entries to this list, as per GamingOnLinux:
· X4: Foundations
could be arriving along with the v2.5 update, due in early 2019.
· Steel Rats
should be adding macOS and Linux builds this Wednesday. At least on Steam...
is still "Coming Soon" here on GOG even though it was released on Steam back on 1 February. It's since added a non-officially-supported macOS build
and Linux is being looked into as well.
· Ashes of the Singularity: Escalation
seems to be working natively on Linux, but its performance is not that good yet.
· The Bard's Tale IV: Barrows Deep
should become multi-OS come next June, in time for the Director's Cut