If his signal channel causes too many scams, users will report and when multiple complaints are sent to telegram support they will review and if true his telegram will be banned and deleted from telegram's system. ..
It is very difficult now to talk about the true reasons for banning the channel, but as a rule, such are the complaints of participants about scam and spam. It is possible that this reason was spam, which was associated with the promotion of this group in the telegram. But we will not be able to find out the true reason, since it will not be in the interests of the OP in any case.