Which Dovecot Pigeonhole Sieve extensions are enabled?

Dovecot Pigeonhole Sieve page lists available extensions, some of which are not enabled by default. I’m looking into writing a Sieve script that would utilize either vnd.dovecot.execute or vnd.dovecot.pipe, which are default enabled “no.” Are they accessible via MiaB’s implementation of Pigeonhole?

Side question: Even better than a Sieve filter calling an external program, I’d love to write a filter that directly hits a webhook. Fastmail allows you to do this with Sieve’s notify extension, hitting a Slack webhook. This must be a custom implementation by Fastmail, and not something that is baked into the Sieve standard, correct?

I’m thinking “no” it doesn’t support capabilities beyond default enabled ones, given this untouched Dovecot configuration in /etc/dovecot/conf.d/20-managesieve.conf:

 # Explicitly specify the SIEVE and NOTIFY capability reported by the server
 # before login. If left unassigned these will be reported dynamically
 # according to what the Sieve interpreter supports by default (after login
 # this may differ depending on the user).
 #managesieve_sieve_capability =
 #managesieve_notify_capability =