I talked to a few of the Discord developers about this and it would seem that the main issue was certain developers not being meticulous about how many requests they were sending. For example, if you have an error that triggers multiple times per second because of some unchecked bug, you would basically be DDoSing Discord with POST requests. I’d say they likely mitigate this by checking certain headers that were present in the request that would reveal it was sent from a ROBLOX server.
There are several very high-profile servers that actively use Discord to log important errors, so I’m going to guess they aren’t completely opposed to error logging on the platform but ban it in the case of ROBLOX due to the very high potential of API abuse.
Then again, some people are still sending webhook messages through ROBLOX; perhaps this is because they’re only going after people causing trouble or they have a certain check setup like “if sent from ROBLOX and channel name includes ‘error’…”
TLDR: Use Sentry, it’s awesome and actually made for error logging.