That is possible. Each user can opt-in. Users >Emails :: The options will be different dependent upon the status of the subscriber. What isn't accounted for is for a subscriber/member to be notified of every post and comment made as that would be spam.
http://b2evolution.net/man/update-notification

There is also the option to subscribe to individual posts

There's no way you should be forcing updates on any European subscriber, as the new GDPO legislation requires an opt-in

(Since 2005 indeed) Users can subscribe to a collection/blog through their user profile and then be automatically notified of new posts / comments: https://b2evolution.net/man/user-coll-subscriptions-panel

Maybe we should have a widget with a "subscribe" button that can be placed somewhere on the page?

Let me know if that would help.

A subscribe button on each post would be useful

@amoun wrote earlier:

A subscribe button on each post would be useful

That one already exists. It's below the comment form.

Oh yes ?? > Click here to manage your subscriptions.

So is the idea is to subscribe to a whole collection and be updated on every post, comment and edit etc or a user choice of those.

Given the https://b2evolution.net/man/user-coll-subscriptions-panel in the back office I'm not sure of what else is wanted. A button on each page that allows subscription to the whole collection?? Can't that be done by selection via the user profile.

By the way I've just clicked on my user name in the evo toolbar > user name, preferences etc on this site and it says I'm not logged in ??

So is the idea is to subscribe to a whole collection and be updated on every post, comment and edit etc or a user choice of those.

All these options are available separately.

I'm not sure of what else is wanted. A button on each page that allows subscription to the whole collection??

Yes that's what I was thinking.

Can't that be done by selection via the user profile.

Sure but it's hidden out of sight.

By the way I've just clicked on my user name in the evo toolbar > user name, preferences etc on this site and it says I'm not logged in ??

Oh I guess this would have something to do with part of the site being on https and part on http. Could you please carefully check your browser address before and after and post screenshots (preferably in another topic)? Thanks.
(If you can also investigate cookies at the same time, it would help. Is your session cookie specific to http or https and refusing to be sent back with the other protocol?)