I just use the SMS Gateway of my provider to send mail to. https://www.liquisearch.com/list_of_sms_gateways

For example, when one of my servers reboots, I have this in cron:

@reboot sleep 600 && echo "Server has rebooted" | mail -s "Alert - Reboot notification" 2125551212@smstext.com

ntfy can send e-mail too, no installation, no token, no signup: https://ntfy.sh/docs/publish/#e-mail-notifications

 curl -d "Server has rebooted" "ntfy.sh/mytopic?email=phil@example.com"

> Without an application what is the difference between a notification and a text message/iMessage?

SMS isn't secure and I can't send an iMessage from the command line. Push notification is the only way to get a message securely to my phone.

> I can see spammers creating schemes to obtain peoples notification URL/address and bombarding them with multimedia popup ads with no ability for the user to block them.

I addressed this in a sibling comment -- it would still go through Apple's servers and there is a lot they can do to mitigate that.

> Tying notifications to apps at least means that there is some audit trail and ability for users to stop receiving notifications and ultimately for the developer/sender to be removed entirely.

If apple ran the service, they would have all the same paper trails and mitigations.