Is it safe to delete a user ?

Simple Answer : NO.

Because then you loose all links to any object to this user: test/requirements/specifications he/she has authored, session he/she has executed...

Before you do you are being warned about this.

The best practice is to mark the user as "inactive ". This keeps him/her  into the database, with all its links.

This inactive user is not counted into your license user limit. So you can replace him/her with a new user at no cost.