tyre wear
for British reg a/c the CAA air worthiness notice 5 I think say 2mm depth of groove over 75% of tread.
also quoted in flying manual.
and in maintenance manual ata 32-45-04.
but some companys allow away from main base the treads to have no tread at all as a get you home allowance, so if is black round and holds pressue it is OK.
which I think is OK as B777 allows in MEL 2 brakes to be locked out, which is the same as a smooth tyre on wet runways.