Aircraft without it are not allowed between FL350 and FL390 on the North Atlantic due separation requirements. Until this month it was only a requirement on the NAT tracks during their published operating times, now it’s a requirement 24/7, 365 for the entire North Atlantic, so unless flights can climb to FL400 before oceanic entry (assuming FL400 is available) then they’re stuck at FL340 or below for the entire oceanic crossing.
In a couple of years, all RVSM airspace (ie FL290-FL410) will require CPDLC.