I have some app/service ideas which all involve "informing" the user about something.

Implementations of this notification mechanism are either spammy, privacy-problematic, or both: (Web) Push notifications, Email, or Messages.

The only solution that doesn't have either of these problems seems to be RSS: Provide the user with (customised) feed link and let them/their RSS client deal with it.

I really wish RSS was less niche and more mainstream. I will "advocate" for it regardless.