Why doesn't SMS spam go to the Unknown Senders tab on iPhone Messages? On iPhone Messages, SMS text messages come through from political groups and all kinds of other nonsense.  These are not phone numbers I interact with.  Some of them are short codes?
Why don't they all go to the &quot;Unknown Senders&quot; tab, since I have that setting turned on?
From Apple documentation:

You can filter iMessages from people who aren't saved in your
Contacts.

There is difference between iMessages and SMS/MMS. You can read about it in Apple documentation: About iMessage and SMS/MMS
Based on that it is relatively safe to assume that if you have SMS (text in green bubbles) then filtering doesn't apply.