They get asked about it all the time. Last time they were asked, they said the forums, r/ffxiv and r/ffxivdiscussion.

The developers themselves watch YouTube a lot, so they see all sorts of memes and videos, and all the negative ones lately since YouTube recommends them sometimes.

Some individuals that haven't been playing lately said they got a survey recently, and personally, I got one in 2021 around the time of Endwalker release aimed at veteran players and it was extremely detailed and had lots of textboxes.

Other than that though, I don't think they are just looking for what one person's opinion is when designing something like jobs. I think their goal has been to reduce friction for the most casual player you could think of - such as a person that plays once every 2 years to catch up on the expansion and its patches, and isn't really any good at the game. Which is why you have things like hotbar buttons lighting up on BLM more, for example.

A lot of job changes have actually been due to widespread player feedback and sentiment as well:
  • The 2 minute burst came about after years of players thinking "why doesn't SE just align the buffs in the first place?"
  • Giant hitboxes came about from people frustrated with lack of uptime on their favorite job. It's a "be careful what you wish for, you just might get it" kind of thing.
  • Homogenization is a result of players complaining (even to this day) that one job in a role does not have something that another job in the role does. "PLD has physical mit, it should work against both" = homogenized. "Dark Mind only works against magic, it should work against both" = homogenized. "PLD should have a gap closer" = homogenized. "PLD is squishy compared to other jobs" = homogenized in Endwalker. These were all player requests... and after years of that, here we are.
As for caring about whether people actually play a job, I think they are interested in people who only play it briefly as well, or only play it to go through the MSQ then unsubscribe. That's why they made Job Gauges, highlight all the next attacks to press, etc.