So, last night I was moving some code around and suddenly I could not deploy my extension anymore, with the error: callback functions are not allowed?
Strange, after a bit of poking around in the interwebs, the best suggestions were focused around error handling on the HttpClient, nothing like that in the area I was working on.
I started undoing my work until it suddenly worked again. Ahh, the offending code was an event subscriber:
The confirm on this particular event needed to be encapsulated by a guiallowed check:
Then everything worked. Not a great error message, and since this is clearly the result of a static code analysis, why not having this as a compiler error instead of the deployment error?