Per a 2016 blog post, Stemma supported triggering webhooks on various repository events a-la Github webhooks.
Do Stemma webhooks still exist? (per the Stemma API packages, they seem to)
If they do, what is the schema of the webhooks? And are there egress restrictions on which URLs Stemma can call?
Using the new custom endpoints feature, could a custom endpoint be registered as a Stemma webhook?