(I only play IF, I don’t write it, so apologies if I’m not supposed in this forum category)
My two cents: if you feel called to write a genderless PC, go for it! There are people, including me, who will enjoy and appreciate it!
My top 2 favorite types of PC are probably (in no particular order) (1) the PC is highly specific and detailed, like a good character concept from a novel. They have a backstory, a distinct personality, etc. etc. and (2) the classic AFGNCAAP blank slate. Note that while (1) often has a stated gender, which allows a lot more detailed backstory, described relationships with other people, etc., they don’t always! One interesting example in this year’s comp I thought was Dr Ludwig, where Dr Ludwig has a VERY SPECIFIC character voice and a very distinct and entertaining personality, but I think is also left gender-ambiguous all game. (And of course what kind of PC you want will vary by genre, if it’s a straight puzzler or if there’s a lot of character backstory-angst.)
If it seems like the game has a total type (2) blank slate protagonist, I will sometimes find that I start to fill in details during my play, either from my own identity or just things that “fit” my understanding of the character. And it mildly annoys me when I feel I’ve been invited to do that except for the reveal 80% through the game that the author conceived of the PC exclusively as a man the whole time. (E.g., you’ve been going along until that point when another character calls you “sir,” you find a piece of mail to you that starts “Mr.” etc etc).
In terms of how to do it, assuming as you say you’re already writing a blank slate protagonist and you also want them to be non-gender specific, it wouldn’t take a lot. Just, as you say, watch any physical description, watch how NPCs refer to the PC (this is why titles like “adventurer” or “doctor” are so useful . . .), watch the terms of address used on any physical artifacts, etc. And sure, we each only have 1 life of experiences to draw on, but I definitely wouldn’t be afraid to experiment with writing characters whose background differs from yours–one of the best parts of fiction is taking on different perspectives, and it would be a more bland world if every author only wrote from their own identity.