Are deactivated leads added to the suppression list?

Yes, if you deactivate a lead, that lead's email address is automatically added to the Suppression List to prevent the lead from receiving future mailings/contact.

If a lead has activity; email clicks, web views, etc, then they will still display in the campaign statistics, so you'll still see the full statistics for your marketing activities.

Deactivating a lead does two things:

1. Adds that lead's email address to the Suppression List, so that they does not receive future mailings/contact.
2. Removes that lead from your contact lists and from the All Contacts view.