unit upgrade from ruins - is there a time limit?

[carloscodex]

Is there an "era" or turn or time limit when unit upgrades no longer show up? I know there was a patch to prevent double upgrades, but it seems to me that after a certain amount of time I no longer get unit upgrades. I sometimes try to strategically save ruins and wait for particular units to upgrade themselves. Later in the game I do not find the unit upgrade.

I decided to brute force try and see what would happen. I am try #68 right now (reload) and no luck. Perhaps it doesn't completely go away but the likelihood of it appearing goes down?

Anyone?

 

[Sweetie Bot]

The latest upgrade I got was a cavalry upgrading into a landship.

 

[Flying Mathias]

I've gotten it as far as the Renaissance. (Musketmen to Riflemen)

 

[Nukes R Us]

I'm gotten 2 upgrades before. TO a cavalry, then to a tank (vanilla version). I was at the industrial era by time I had upgraded again to a tank, but still. I got it