Don't you have auto save enabled to reload a previous save?
If you talk about returning to a saved game BEFORE the 999+ bug occurs, the answer is simple : an hour before I posted here, I thought the 999+ was a new event in Frontier Pass, and not a bug!
Something like a limiter for huge civilizations, and it is always my case...
In my games, I usually win before the faulty policy card is triggered, hence I did not worry about the couple of times where I saw the 999+ toward the very end of my games.
It is especially true at deity difficulty that I always play for years now: and as the dark age mode is enabled, it is VERY easy to win by simply being patient: the AI tends to over-build, and as soon as a Dark Age comes, the cities rebel and it is a cascading event. Even big civs are destroyed. Self-destroyed. You just need to hold on your few city very close from each other and let the storm goes away, then the competition is gone...
Now, if you talk about the CTDs/end turn freezes:
Of course I have, keeping the last 100, all turns. When you play deity with all game options on, I am not good enough for surviving without restarting many times and I use and abuse the Time Machine...
And it is what save my day when bugs are invited into my party, and it allows me to proceed with my games for the last 6 months...
Just before hitting the end-turn, always manual F5 quicksave, and if Freeze or CTD (direct to 3) :
- - Ctrl+Alt+Del
- - Task Manager (with option "always on top"), then kill Civ 6 process
- - Wait a bit for steam/firaxis cleaning then reload the launcher + game
- - ... and check my latest save: as I indicated in other forums (reddit + steam community), I find 99.99% of the time the save game AFTER the frozen end-turn. By instance, if I end the turn at 600, I find when reloading the automated saved game for start of 601... and it loads properly and I can continue my game sometimes for hours...
It means that the end-turn does work (most of the times), but a process at the end blocks or fail to return a green light...
I only have a CTD 2 or 3 times this last month, and freezing end-turn perhaps 80 times or more.
And only 5 or 10% of the time, when ending Civ6 process, I can also see the sub-process Firaxisbugreporter.exe, but stuck, and only 1 time (in perhaps 50!), it asked me to send data to Firaxis, which means that bug reporting did not work as intended and sent nothing most of the times as I also monitor internet traffic... but 50% debug data is sent to Microsoft.