https://wccftech.com/ios-13-3-includes-improvements-to-minimize-junk-messages/

With iOS 13.3, Apple has made improvements to help minimize junk messages. The aim is to reduce unwanted messages, however, Apple has not made it clear in the OS how it really works. Thanks to a new support article on Apple's website, we have more details on how you can make sure that your messages don't end delivering as junk to others.

If your messages are marked as junk, they end up in the recipient's Unknown Senders list in Messages app. To avoid from this happening to your messages, Apple has shared the below details in a new support page: