Using the Rest API, I can retrieve both task IdentityLinks and HistoricIdentityLinks (and maybe process instance Identity links, though these appear to be from the task package).
Regardless, the pojo interfaces both use groupId and userId. Most of the Rest docs incorrectly show responses of groupId and userId also.
However, the RestIdentityLink has group and user attributes which ends up being marshalled to JSON. The HistoricIdentityLink uses groupId and userId, which I assume is correct since you'd want to follow the engine API models.
Is this a bug? I don't see anything in the Git history suggesting it was changed for one reason or another. The docs clearly show userId and groupId, though in a few cases user and group (in the processInstance IdentityLink responses). This is the 5.19 docs, though I'm using 5.20.
The master branch (5.21-SNAPSHOT) has not changed either.
Thanks,
Bill