Deactivated classes are hidden from the main list but still available via the status filter.
On the Classes page, the toolbar has a status dropdown. It defaults to "Active classes". Switch to "Inactive" (with a count) or "All classes" to see deactivated ones.
Members can no longer book the class from anywhere (booking page, member app, staff booking, drop-in, standing weekly bookings) and it stops repeating. Upcoming sessions nobody has booked come off the schedule. Sessions people already booked stay — you are shown which ones, so you can open each to cancel it (members are notified) or let it run.
Open a deactivated class to reference its template details, duplicate it, or reactivate it. Reactivating puts back the upcoming sessions that deactivating took off the schedule.