The keyword in your sentence is customers. The people using Facebook aren't its customers, they're the product and thus open season for all kinds of nefarious things you'd never do to your customers.
I think the “user is idle let’s poke the user” is a supposedly a “nice” user engagement to get users back on the system. I hear your concern.
I think at this point:
* opt out notifications before deactivate your account (go to account settings)
* deactivate and opt-out notification at the time of deactivation
* set facebook email as spam or something (although you’d miss any potential real security notifications.)
Or you can also remove your phone number and change the account’s email address. I own a dozen gmail accounts, one for mailing list, one for billing, one for personal, one for school, one for important tech accounts like digital ocean/aws, one for banks, one for general garabge I don’t care and so on. I also run a YouTube channel with a friend so I also own a separate gmail account to be added as a manager of the channel.
Much neater than labels (amazon emails can fuck me up easily, ugh).
I highly recommend this multi-email account approach. I can ignore 50 emails sitting in other accounts most of the time. All of them have second auth and some accounts have different passwords for additional security. Not hard to manage because the address format is always <A>.<purpose>@gmail.com. Consistency!!
The only major issue I've had, with gsuite at least, is not being able to reply from one of the not-actually-existent addresses on my domain.
If they don't provide the unsubscribe option as a one-click option, mark their emails as spam. It's harsh, but getting flagged for spam when they won't stop spamming you or providing the required one-click opt-out is their problem, not yours.
I've no intention of logging back in to check my notification settings, but when I was using Facebook, I would get a push notification maybe once or twice every couple of days. Now, I get more than a dozen SMS messages a day.
I've built notification systems in the past, and in every instance, respecting the fact that abusing the notification power entrusted to us by our customers was seen as a cardinal sin. Maybe it's the absence of that respect that has catapulted Facebook to where it is, or the presence of it that's held me back in my own endeavors, I don't know, but man, as the recipient of all this spam, it really, really feels unsavory, and further evaporates whatever trust I might have had in the platform.