I don’t have any statistic, but I would guess that the proportion of people wanting a Jack is significantly higher in the group of people interested in buying Fairphone that on the general population.

Fairphone literally does have that statistic. They spent effort to gather that info in order to inform their business decisions. And they report:

We also looked into the consumer data and Fairphone 4’s weight and thickness were more of an issue than the lack of a minijack

My guess would be yes, the whole point of including a legacy port like this would be compatibility. Laptops used (still use?) PS/2 long after desktops moved to USB keyboards because the lack of polling meant improved battery life, so it’d make sense for the hardware support to still be there.