Is it possible to filter Task and Process by check if a variable exist or not using the REST API

In our system we have a created usertask in Camunda that require a user to assign someone to do a task. In a separate page of our application we pull all usertask that are an assigment task and that has been completed (finished) to be able to see when the assignment has been completed and to whom is now in charge for the next part of the process. But we now can cancel process and don’t want to retrieve it so we have added the Process variable : CANCELLED.

So we now can filter out all usertask (assignment or not) that come from a process that has been cancelled : /history/task?processVariables=CANCELLED_eq_false

but now all previous task are not retrieve anymore, so we didn’t have any choice but to go in the camunda database and add manually the variable to all previous process.

Is there something like cancelled : /history/task?processVariables=CANCELLED_nexist

Anyone having any ideas ? Is it a missing feature ?