To be fair I don't think this post is about sexism or inequality. I take it on face value that it's about using the proper pronoun. I also think there are different levels of formality and expectations for addressing people within differing cultures and locations.
For me I would simply use their name or "they/client" if appropriate to skirt this issue. Of course I am coming at this as an American on the West Coast. Head to the East Coast and you'll see even newspapers like the NYT say in articles, "according to Ms. Weiss..." or "Mr. Stevens then indicated...". Other countries as well have a more formal approach when speaking to or about people they don't know, people who are older, people who are in more senior positions, etc.
So a question might be whether a site like Upwork should try and accommodate these different cultural or location based differences or should we come together with a homogenized approach that defines an Upwork culture. The answer is probably to accommodate what reasonably can given this is a global platform but you cannot be all things to all people and so lines need to be drawn. For me, indicating gender is a bridge too far. My opinion is that adding a "Mr" or "Ms", etc. would not be received well by the majority (as defined by over 50% of the people). Certainly any sort of gender designation (i.e. male/female) should be a complete non-starter. So this means that Upwork is going to be a less formal platform but of course that doens't require anyone to operate at a certain level of formality in their own writing. I grant you for the gender uncertain issue that the OP is asking about this may not help, so I'd say you'll have to work around that as best as you can for what I imagine are the minority of cases.
