It's a red flag? Come on, if I wear a suit it's because I want too and has no impact on my skills as a software engineer.

Being hyper judgemental about the clothes people wear isn't productive

I would go as far as to say being this hyper-focused on clothes rather than if the person is sociable and competent is a red flag itself. It is rather superficial. Vague platitudes about "culture" might get thrown out, but are we engineering and building things or are we putting on a fashion show?

Calling it a red flag may have been too harsh. It's certainly not an immediate no.

However, like it or not, it is a signal because it means you deviate significantly from the mode of the distribution. And a sober application of Bayes suggests that if anything, all else equal that signal is a negative one.

I've gotten a job offer from every technical interview I ever took in a suit, so it Worked For Me. And none of the jobs that I took I ever wore a suit to again (except for conferences or trade shows, and occasionally when I was going out after work to somewhere posh, which did provoke fun "Omg are you interviewing" questions!) Which I actually have found a bit of a shame because I do quite like a chance to wear a suit, though I'm also grateful not to have to iron infinite shirts.

Admittedly I thankfully wasn't in the SV bubble where people are wound this tightly about it!

An interview is not a regular work day. If only things relevant to the job were required in an interview, no one would be talking about whiteboard exercises.