I don't have a "backup GPU" for my main system, but I have an entire backup computer on my KVM only a few button presses away. That computer uses 3x GeForce GTX680 in SLI. Aside from not having DX12 or Freesync support, the 3x GTX680s still do very well in the games that I care about.
The 680s carried me along for many years. I got the first one as a replacment for the 2x 4870x2 in quad-crossfire that I was running before. Then I got a 2nd GTX 680 not long afterward. I ran that 2-way SLI setup for a few years. Right about when I was going to replace those with a newer single card was when the first crypto mining boom occured. Instead of paying the inflated prices for a new card, I just got a 3rd GTX 680 instead. Used that triple-SLI setup until I got my 2080.