Just for a bit of extra detail on how it currently works. 
For non-staff, a suspended user doesn’t show up in the @mention autocomplete that pops up when you start typing straight after the @, which may be helpful to dissuade regular people from trying to ping them. 
And if you don’t hide suspension reasons, their user card should look a little like this when clicked on:
That’s if I suspend them ‘forever’ and add a custom ‘no longer a member’ reason.
If you only ever suspend users due to expired memberships you could even amend the respective site texts and have it look more like this:
For that I updated js.user.suspended_permanently and js.user.suspended_reason, and added the date using the custom reason option when suspending the account.
Though you could go one step further and also deactivate their account as well. That would then hide their user card for normal users and replace the info with ‘this user is no longer active’:
So if people do manually type the ex-member’s @mention even though it’s not in the autocomplete menu there are still a couple of ways you can let people know they’re no longer an active member.