I want to test ẗhe Maladex Rewards calculator but they ask me to enter my rewards address. Is there any conceivable risk in doing so?
The only risks associated with sharing any kind of public/verification keys or addresses have to do with de-anonymization. If you are concerned with Maladex, your ISP, a traffic sniffer, or any other entity possibly linking your rewards address with your IP address, then you might have something to worry about. However, assuming you aren't massively evading taxes, living in an oppressive regime, or have some other target on your back, there is very little risk to your funds when sharing your rewards address.
If however any of the above do apply to you, or you prefer to stay on the ultra-paranoid side, there are still options. You can use cardano-db-sync to query chain information from your own running node, including the reward history for a given stake address.
Another option is to learn how to access the internet in a truly anonymous fashion, and use third parties like Maladex that way. Warning: this is a deep dark rabbit hole, not for the faint of heart. If you are interested though, I'd recommend starting with The Hitchhiker’s Guide to Online Anonymity.