They were acquihired by Zynga of all companies which then promptly proceeded to kill off all their products.

GTA IV's tech was quite ahead of its time (2008), with the integration of Euphoria (which had to be integrated in-house by NaturalMotion engineers inside Rockstar's studio @ Edinburgh). The other comment correctly pointed out that NaturalMotion was acquired by Zynga, who in turn got acquired by Take-Two (parent of Rockstar Games).. So who knows, perhaps we'll still see an updated version in the sixth major instalment of the franchise.

Euphoria was used for a subset of animations, for the most part traditional mocapped animations (and a lot of them) were used to drive character movements. I don’t know if motion matching was used on GTA4 but it was on other Rockstar games at the time and subsequently.

Characters blended to the hand written, custom, euphoria behaviors in certain situations… it handled falls, stumbles, deaths, etc. Generally it replaced ragdoll physics but wasn’t used during most character movement behaviors.