Depends on the context. When written: MSFS where I expect people to know what that means, Microsoft Flight Simulator where I don’t. Always the latter when spoken.
When it comes to writing/typing it I use MSFS 2020. When I say it I do it like “MussFuss 2020”. Btw I’m still waiting for your FSFlyingSchool to come to the official MSFS marketplace
To differentiate from other flight simulators i say MSFS (there are more than one, really, look)
To differentiate from FS2004, FSX, FS98 etc, i add “2020”
Some people on forums just star talking about flight sims and they generally refer to XP11. Its like you wandered onto a gaming forum and started talking about “the game”.