The email protocols don’t natively support this, so yeah your best option is to embed or link to a webpage that becomes inaccessible after a set amount of time.

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Which is also pointless - “legitimate interest” is basically them saying “i have a very good reason to access your data, as per GDPR, so I don’t need explicit consent”. Deselecting it has no effect, I think they only do that to make you lose time. The only way to avoid those companies using your data is to challenge them in court and show that they do not, in fact, have a legitimate interest. That’s their whole game