I see, the idea is already out there, see note15g67u2cj406dn7r7ya3dm2rk8mnj4akj99ay260ek60ctr66k5ws5v5km6 for example.

So I prefer to link conversations here instead of GitHub. Let me sketch the idea:

After key generation you would publish one or more spare pubkeys with a special event id. They are the canaries. Clients can look them up to see whether they published other special events (or type 1 even). You could leave it up to further NIPs or clients what do do if a canary tweets.

The advantages of this are that it‘s fully compatible with currently existing pubkeys and that clients don‘t have to implement them for most things to work.

One problem I see is time. You would either need to be referenced by something newer but old enough or you would consider a canary only valid for checking after a fixed amount of time.