We are thinking of dropping the MIR backend from GTK 3.24.
Apparently, it hasn’t been built since 2017, as one of its dependencies hasn’t been updated; the MIR backend hasn’t been built on Ubuntu since early 2018 because of that either.
I suppose people using Ubuntu LTS before 18.04 may still use it, but those won’t be affected by a change in GTK 3.24 anyway.
If people are using the MIR backend and GTK 3.24, this would be a good time to speak up.