What constitutes to ‘earlier’? I myself have tried using post requests to a discord webhook around two months ago and have gotten the same HTTP 400 error.
The devforum is also littered with posts as early as 2018 with people reporting that using discord webhooks in conjunction with HTTPService will return the aforementioned error.
Discord doesn’t block direct post requests from roblox. They had a period of time where they did though from people abusing the post rate, but it still works. I just tested it to be sure a few minutes ago and it went through properly and didn’t get any errors.