You may have noticed two new checkboxes in Clients > Details: Active, and Marketing email opt-in.
By default, all of your clients are marked as Active. You can uncheck the box to mark inactive clients and hide them in your Clients > List without having to permanently delete their records.
The marketing email opt-in is a field you can add under Setup > Booking Details to the form your clients fill out. It lets your clients indicate whether they'd like to receive promotional messages from you. Then if/when you send a mass message from Clients > List, you can filter to only those clients who have explicitly opted-in.
In general, it's a good practice to ask permission before sending someone bulk email. But if you haven't added the Opt-in field to your booking form or a client hasn't made an appointment with it in place, they wouldn't have had a chance to opt-in and the box will be unchecked. Note that you can also ask clients in person and then check the Opt-in box yourself.