We have a couple of metrics available around revocation mentioned
here (if you search for "revoke"). In a Vault response containing a token, Vault also returns information about when the token will expire, which you can find some examples of
here. Also, if your Vault log level is on "Info",
this message is generated when a lease is revoked. It may be possible to alert based on a particular log line appearing using something like ElasticSearch alerting.
Those are the present methods - we don't have an event service that's pushing out notifications to web hooks.
What's your use case? Just curious.