Since MySQL 5.7.8, user names can be up to 32 characters long. I have searched on ISPConfig to avoid this limitation with MySQL 5.7.8 or higher, and I found the 16 characters limitation is in file database_user_edit.php, so changing the limit to 32 is working. It would be nice to allow support for 16 or 32 characters, depending on the underlying database (or a parameter in settings). Related post: https://www.howtoforge.com/community/threads/configure-ispconfig-max-database-username-lenght.53557/
Please make a feature request in the bug tracker, otherwise the request might get lost You can find the bug tracker here: https://git.ispconfig.org/ispconfig/ispconfig3/issues