Neutral tag should not be a problem for joining signature campaigns, it's just showing what happened in the past with that member.
I doubt anyone will remove their neutral tags for him, but this is not the same as having negative tag and he was not accused for scamming anyone.
Even though you're right, I would say that when a user gets a neutral tag that says a user is either simply a spammer or an AI-spammer during the time they are in a paid signature campaign, it basically says that they have been misusing the opportunity by spamming around the forum only to make money from the campaign and haven't been producing anything much organic. So basically, it's not only the scammers that shouldn't get slots in signature campaigns.

I know OP has been a very good user in the past, and he isn't generally a spammer, but he did make a mistake by using an AI model to make posts while being in a signature campaign, and I guess this is his punishment for doing that. He wouldn't have any problem, and this discussion or thread would have had a different perspective if OP had created it before doing the so-called experiment, as he calls it, instead of creating it after he was caught and reported by users for making AI-generated posts.
