I used to think this was a good idea, but I see so many resumes done in TeX that they cease to stand out.

Of course, just throwing TeX at your resume isn't enough. However, TeX gives you tools at an engineer's level which can be used to make your resume stand out.

Is that more of a recommendation or a hard requirement? I've been using a bland resume that I thought would check all the boxes inside an ATS.

Just a recommendation. TeX is a very engineer-oriented design tool. An interesting way to spruce up your resume.

It's a very good recommendation. Simply being aware that TeX/LaTeX is a thing (it's very recognizable) is something that I tend to appreciate in an engineer.