With substitute teaching, substitutes are considered disposable. You're there to fill-in for the day for their teacher. There are many reasons why substitute teachers are excluded from schools -- poor classroom management is the #1 reason why a school would ban a substitute teacher. The #2 reason is that the substitute behaved inappropriately with a student or class of students.

Employers usually tell an employee why they are being terminated. That's par for the course.

Since you said it was your favorite school, I can understand why it would bother you 4 years later why they suddenly excluded you from subbing for them. Maybe you said something to a student and that student reported you.

Or, maybe the principal thought you didn't have good classroom management skills. It's really hard to say. You'd have to contact the school to find out. If doing that will ease your mind, then just send the principal an email.