technically it is “not supported” anywhere. However, plenty of users use it in 2.4 and I can’t think of any changes off hand that would have broken it in 2.5, although it is possible.
I can now verify that cusctom contexts are working.
I was expecting that the select-box custom context would show the selected context as selected after saving.
Instead it copies the context to a text field 13 rows above the custom-coitext select field.
So it was more or less a user error…