I learned to read and write in the 1970s and 1980s and misusing "they" in a singular context would result in a red mark on one's paper and a lower grade.
Any time I read a sentence that uses "they" as a singular it's a small mental roadblock. It's like stumbling on a displaced brick in a sidewalk. If I read "a good programmer will work to understand the problem they are trying to solve" I will mentally trip over that and be distracted from the point being made.
If I don't know whether my subject is a male or a female person, I will use "he" as the singular. "A good programmer will work to understand the problem he is trying to solve."
I learned that "he" refers to a male or is neutral, but don't mind when writers use "she" as long as they are consistent about it. In a paragraph about the habits of a good programmer, flipping from "he" to "she" every other sentence is also distracting.
Also OK if the writer avoids the issue: "Good programmers will work to understand the problems they are trying to solve."
Yes I realize that the usage of "they" has evolved, as language does, but for me it's hard-coded in ROM at this point.