According to the final comments in SC#65:

According to the final comments in SC#65:

Made isteacher() require that the first parameter (course id) be
specified and non-empty. If it is empty, [i.e., 0, which was used to
simulate what has now become isteacherinanycourse()], then the return
value IS correct but a warning is printed on screen. This should allow
us to track down any such calls in legacy modules without breaking Moodle.

The correct way to check for teacher status in ANY course is now to
call isteacherinanycourse().