External/Available without login Suitelets do not accept POST requests anymore?

I have a webhook running and its currently receiving post data

It works. But you would better to use RESTlet instead of Suitelets for accessing from outside of Netsuite.

You might have to spoof the user agent

@michoel I found that, my external system, the one I don't have control over does not have an option to set the User Agent.

You will probably need to use middleware/proxy