Can you name another corporate email provider that doesn't free up seats when users are deactivated? To my knowledge this is how all of Proton Mail's competitors charge for seats - at least all the ones I know of.
I haven't used many "corporate email providers", whatever that means, but I've used my fair share of "classic" email providers (standard mail stack + basic webmail) and most of them charge per inbox. If an address is deactivated, but the inbox is still present, it counts towards your quota (depending on your package, this might mean you're paying for it). If you want to reclaim it, the inbox with all its contents must be deleted. Since that is a highly destructive and usually non-reversible action, a few do require you to go through their support.
I personally don't like it and surely there's better ways of doing it, but it is definitely not unheard of.
