I honestly think that suggesting that dear is gross or condescending is a bit of an exaggeration.
I’ve never known the term to be used in the West in anything but an informal polite way. It has never made sense using the word in a formal sense or to imply anything like a gender stereotype.
I think it needs to be remembered that people using the word ‘dear’ on Fiverr are usually from former UK colonial outposts.
These are places where in many cases, British English became the official language in very short space of time. In this case, the term ‘dear’ being used in official documents like “Dear Sir, your tax is due,” likely gave many people the impression that ‘dear’ is/was a civilized and posh way to start correspondence.
In short, the term is not derogatory or (in my opinion) ever used to infer anything like a prejudice toward anyone of any gender. If anything, it is a linguistic quirk.
Of course, it is still not appropriate to use in a professional sense on Fiverr. However, I think it is a bit of a reach to imply that it is offensive. If anything, you could argue that it is offensive for people to be so outraged that people from other cultures aren’t as hip with latest Western communication norms.