I read somewhere that GDPR requests for restricted processing (Art.18) cannot be combined with any other topic or request. E.g. If you request that they not use your e-mail for marketing purposes.

WTF. Yes, I understand the idea is that if the request stands on its own, it cannot be overlooked. But #GDPR requests are ignored so often that I deliberately combine a GDPR request with another request that is more difficult to ignore. That way when they ignore the GDPR request but treat the non-GDPR request from the same letter, it proves that the data controller received my letter. When a GDPR request is made on its own, they can more easily claim the letter never came and shift the proof-of-delivery burden onto me.