I have an important update.
I’ve done some more digging today and this is to the best of my knowledge the situation:
We have access to the non beta (production URL). I can create subscriptions on /1.1/account_activity/webhooks.json without issues, just not receiving events.
I think the reason this worked last summer but not now is because the Account Activity API was moved into the Premium API category and put into a beta mode sometime last fall/winter.
We do not have access to the beta environment (‘env-beta’) for the following application IDs:
125311 – Our Production App
1467830 – Our Staging App
Let me know if there anything I can help with to make this process easier.