It's interesting how we've looped back to finally having technology that allows us to have mirrors in games again.
In the forward-rendering days of the 00s, there actually were some mirrors (working by essentially re-rendering the scene), but of course it was just mirrors that were reflective and they were specially built.
Then over the 10s, we got more and more realistically reflective surfaces, but what they reflected was usually a relatively crude approximation derived from some combination of precomputed and screen-space data. Enough for shiny materials, very bad for a mirror.
Now, in the glorious 20s, we can get actually mirrors and reflective surfaces. Whew