If I take a step backward, my current need isn’t about a comment webhook itself, but more about a notification system, you’re right to rephrase it. I hope however that implementing such webhook isn’t too difficult, since I think it could really bring a plus to Bsr if people want to implement their own notification system with 3rd parties tools like n8n/zapier.
Notification could mean different implementation :
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sth like a
icon on the top of the dashboard of each user. When a user mention another, it changes the icon with a little bubble with the number of times he/she was notified (similar to the way we are notified on chat apps like Telegram, Signal etc.)
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email sending : the notified user received a mail containing the link to the row on which he/she was notified, with the comment and the user that posted it
Both would satisfies my case 
Side notes - Looking at airtable webhooks & notification stuff
Strangely , I find the Airtable webhooks quite scarce, there’s no sign of comments in them
They however have a strong notification system : Managing Airtable Notifications | Airtable Support like the one you are suggesting