Currently the KLC requires a clearance of 1mm around BGA devices. As i don’t have access to industry standards, i can’t check where this requirement comes from. Does it apply to all BGA devices regardless of size? (1mm clearance for a 0.8x1.1mm device is quite a lot.)
Another maintainer mentioned that the large clearance might be necessary to allow for inspection of the balls.
A bit of searching and i found this https://www.pcblibraries.com/forum/1mm-pitch-bga-courtyard-nominal_topic1752.html
From that i would guess 1mm is the clearance for nominal density. (In the lib we use nominal density as our guidline.) But the answered question there was about 1mm pitch bga.