While there is a webhook action for updated contact, there seem to be non for changed segment membership? I was looking at contact updated, but it could logically also be related to segment updated?
(this support item seems to be related)
There is no segment update webhook currently, but this would also be difficult. Currently webhooks seem to send the full record, not the change. Sending all the segment members for a segment update would probably not work well.
So this should probably part of “contact updated” webhook?