The following rare student writing example shows how ‘used not to’ expresses modality:
PELIC Korean female level 3 grammar class.
*In other words, she did not have permission to drink coffee. Although now she is an adult and can.
Point 227 in C2 MODALITY is defined as:
negative form ‘used not to’
This is a very rare grammar pattern in iWeb:
1 USED NOT TO BE 57
2 USED NOT TO KNOW 12
3 USED NOT TO MAKE 12
4 USED NOT TO DO 10
5 USED NOT TO HAVE 9