(*conspiracy theory warning*) I think DSP may have added certain phrases to the banned words list to block certain usernames/try to stop the long name spam trolls. I'm referring to the youtube blocked words list, not the nightbot banned words list.
Some time ago I had an account with the words "WWE Champions" and "DownFromTheRafters" and none of my message would get sent in his chat. But on another account that didn't have those words, my messages would send just fine. Also if I remember when I changed my WWE Champions DownFromTheRafters username and removed those words, my messages would get sent in his chat.
I could be completely wrong, maybe the account got shadowblocked by youtube for some unrelated reason. The test message I used was just "hello" and it was the first message I sent on the WWE Champions DownFromTheRafters account so I didn't use any banned youtube words in my message. Also my youtube privacy settings have it so my subscriptions are private so I don't think DSP could have banned the account ahead of time.
I guess the way people could test it would be to have an account named something like "DownFromTheRafters WWE Champions" or "DSP's account is DownFromTheRafters on WWE Champions" something like that. Then sent a simple test message like "hello" and see if gets sent. You can refresh the chat or have it open on another device or something to see if your message got sent. If it didn't get sent, go to your profile and change your username to something generic like "ryan" or whatever and then send a test "hello" message and see if it gets sent. If it gets sent after changing your username, then I think it's probable that he has added those words to the banned words list. (remember to unsub when you are done with the account)
Not telling anyone to do anything, but if you want to make additional accounts on youtube, just click your profile icon on the top right, click switch account, then click view all channels. Then click the create a channel button. Oh and after you create the account click your profile icon, settings, privacy, and set subscriptions to private. (also btw if you sub to a ton of people on youtube, creating multiple accounts like this is a good way to spread out/organize your stuff across multiple accounts instead of subbing to dozens of people on one account and getting your subscription feed spammed by everyone). Apologies in advance if my sperg theory is wrong. It doesn't really matter but I just posted to this because I'm curious now if he has added phrases to the banned words list, so now certain account names just can't send messages