For me, that limit is “none”, I have stopped paying money to several services I used regularly the first time the mobile app gave me an unsolicited notification.

The upper limit is still zero.

No. Most customers will tolerate quote a lot of this form of advertising just like other forms. Enough to make it profitable to spend significant effort on it.

Whether customers will tolerate it has no bearing on whether it is acceptable to bake an antifeature into an application.

In your opinion. In marketers' or app developers' opinions, it continues to make them more money so that's what they'll do, regardless of whether a small minority of consumers complain about it. Of the people I know, only technical people have turned off notifications, most average users simply have long lists of notifications on their phone, that they may or may not look at, ironically making the value of each notification from some random app a lot less.

Sometimes I feel like a space alien: I didn't watch an add since 2008 when I divorced and didn't need a TV any more.

I'd rather not use a service than to get adds. The day YouTube stops me from using an adblocker I'm going to have a lot of free time on my hands!