I worked with browsers since before most people knew what a browser was and it will never cease to amaze me how often people confuse slash and backslash, / and \

It’s some sort of mental glitch that a number of people fall into and I have absolutely no idea why.

I doubt those very people would confuse the two when presented with both next to each other: / \ / \. The issue is, they're not characters used day-to-day so few people have made the association that the slash is the one going this way / and not the one going the other way \. They may not even be aware that both exist, and just pick the first slash-like symbol they see on their keyboards without looking further.

I wonder if it's dyslexia-adjacent. Dyslexic people famously have particular difficulty distinguishing rotated and reflected letterforms.

Could be. The frequency is such that it could be dyslexics. It's not all the time, but it's a steady rate of incidence.

I think in the 90's it was just people repeating a pattern they learned from Windows/DOS.

It used to grate on my nerves to hear people say, e.g. "H T T P colon backslash backslash yahoo dot com".

But I think they always typed forward slash, like they knew the correct slash to use based on the context, but somehow always spoke it in DOSish.