I’m sorry, but Windows 8.1 is from 2013 (the release year of Discourse 1.0) and has been officially end of life for two years now .
Macos 10.14 is more recent, but its successor Catalina runs on all Mac devices released since 2015.
You simply have to draw a line somewhere.
Discourse has had high browser requirements since the start. Here’s the line from the announcement of Discourse:
Designed for hi-resolution tablets and advanced web browsers.