There are two applications that collectively people might call "FAF."
The first is the "client." The client is what you use to connect to look for games, host games, chat, download maps/mods, pretty much everything other than actually playing the game. The current version of the client is v1.6.0 "downlords-faf-client.exe"
Then there's the game itself, "ForgedAlliance.exe" and associated data files like units.nx2 or whatever they're called. When you join a lobby, or when you host a lobby, that's the "ForgedAlliance.exe." And when you launch a game from the lobby, it's the ForgedAlliance.exe that handles everything (graphics, user interface, keeping track of units, calculating projectiles, connections between players, in-game chat, etc. etc.). The client basically stops doing anything related to how the game goes, whether it's stable, whether it crashes. The client might have a role in terms of collecting information from the game, like telling the replay server who won and who lost, but the client has nothing to do with the crashes. When the game crashes, the client can report to you that a crash happens. So you might see an error message coming from the client and think that it's the client's fault, but actually the crash happened in "ForgedAlliance.exe"
If you wanted to play an older version of the game: first, as Sheikah says, everyone you're playing with would have to have an identical version loaded. To do this you would have to use old versions of the data files, which wouldn't be impossible to set up. Any time you watch an old replay from the vault, it has to load the correct version of the data files in order for the replay to display properly. So it's definitely possible to collect the old versions of data files and use them.
If the devs can't get the crashes under control in the next month or so, then it would make sense to revert the changes, or to add a new "game type" (for example, "nomads" and "phantom-x" are game types) called "FAF 2020" that used a stable version of the game. But I think it's too early to take drastic measures like that.