Recent usage of the nLockTime field is by MARA and f2pool (as your graph shows). They use(d) it to store metadata about jobs, to make sure a disconnected miner can resubmit its shares upon reconnection. Following my email to the bitcoinminingdev list last year, MARA updated their software to use the coinbase’s nVersion in place of nLockTime. F2pool could do the same but my takeaway from personal exchanges with them is that they don’t want to bother until BIP 54 is further along the way toward activation. (Which, of course, is a nice catch-22 in itself.)
But as far as incompatibilities go, all pools are currently incompatible (modulo one recent change –see next post), and i am more concerned about the swarm of tiny pools that currently hardcode their nLockTime to 0, than about easily updated practices from a few more easily reachable big pools.