@Andrej Weigandt Thank you for reaching out.Yes, it is supported for customers to provision read replicas or replicate from on-premises to MySQL Flexible Server with data-in replication.
https://learn.microsoft.com/en-us/azure/mysql/flexible-server/concepts-data-in-replication
https://learn.microsoft.com/en-us/azure/mysql/flexible-server/concepts-read-replicas
Regards,
Oury