M0.84 to 0.85 is normal cruise speed
M0.86 to 0.88 is possible but heavy on fuel. OK for a short while.
M0.90 is Mmo.
Although it is possible to fly at Mmo, any exceedence (caused by windshear, wave or auto flight misbehaviour) would trigger an over-speed warning. The crew would then have to report it and explain what they were doing.
The crew may have pushed up the Mach number briefly, maybe up to M0.88 to M0.89, just to see a high ground speed readout.
There is no readout of skin temperature, and therefore no limitation.