An issue would be something Xkcd joked about

A less agressive approach imo is to have RSS-tracking instances and bots. It is not uncommon, that I can observe, for smaller sites to have those, and by leveraging trackers, you at least can keep track of those without having to push against the majority of the internet.

When you say linking identity, what do you mean? Validating that different accounts on different services belong to the same person, or linking for authentication, or something else?

For the former something like Mastodon’s verified profile fields (e.g. homepage) could be used - if one uses the same verified field across services, it shows that they all belong to the same person.