In the browser I can query "assignee was currentUser()" and get results just fine, and when my company was using jira server this JQL also worked just fine through the API, but now that we've moved to jira cloud I'm getting an error "A value provided by the function 'currentUser' is invalid for the field 'assignee'."
Looking at the documentation it does indeed say that "=, !=" are the only allowed values. I'm just confused as looking at the old docs it also said "WAS" is not allowed but it worked then, and now it doesn't work through API but does work in the browser.