A good enough and low hanging fruit solution to spam is an allowed list. Generally allow contacts (initially at least). Track spam feedback by age against contacts.
If someone does end up in a spam list (and they don't rack up a high score across multiple targets), let them know they're in such a list and where to start looking to resolve that issue. A good enough solution for this is to have number carriers attest to have verified the government issued ID of the individual in question; and if spam happens shortly after that to yield the government ID number of that individual.
An alternate form I've considered, for email, is to pay a postage (transfer + storage) micro-transaction fee, and possibly an attention fee for prompt review. The custom might be to refund these in cases of legitimate messages.
I have received zero spam messages over iMessage in maybe ten years, without using an allow list. Why, as a consumer, would I tolerate the degree of effort you describe when I have a perfect zero-effort solution available today?
If someone does end up in a spam list (and they don't rack up a high score across multiple targets), let them know they're in such a list and where to start looking to resolve that issue. A good enough solution for this is to have number carriers attest to have verified the government issued ID of the individual in question; and if spam happens shortly after that to yield the government ID number of that individual.
An alternate form I've considered, for email, is to pay a postage (transfer + storage) micro-transaction fee, and possibly an attention fee for prompt review. The custom might be to refund these in cases of legitimate messages.