Preferences Color Management option to select soft-proofing profile is missing in GIMP-2.99.18
Open GIMP 2.99.18. Go to Preferences/Color Management. Scroll down to where the user used to be able (still can on latest GIMP 2.10 from Debian Trixie) to select an ICC profile for soft-proofing. That option is missing in GIMP 2.99.18 compiled today on Debian Trixie. It's also missing in the Windows version of GIMP 2.99.18 downloaded from GIMP.org.