You don't even need to back up the whole directory... There are only 3 files that need to be stored —with the addition of a 4th one as we just found out— to roll back your game to a previous version: GTA5.exe, GTAVLauncher.exe, update.rpf and steam_api64.dll. There was a time when you needed to have the right version of Rockstar Social Club but I don't think it is a thing anymore. The largest file to back up is hardly half a GB...
I have been playing with RPH 0.45, that's from August 2016 (!) almost every week since then, I have gone through all the major updates including this last one without a single issue to roll back to the version of GTA in that summer. All I am backing up is the 3 files in bold character above and the various DLL files and crap from the main directory, less than 1 GB in total.