When a user is removed from my G Suite domain, is his data still available in the on-premises backup?

Yes, the deleted user’s data is always safe in the local backup.

Restore to another G Suite account

You can safely restore (merge) all the G Suite data for this user to another user. For example, if an employee is leaving your company, his G Suite data can be merged with the employee who is responsible for taking over his job. Detailed instructions can be found at Tutorial: How to transfer G Suite data from one account to another.