It's a reasonable hurdle; an employee who previously signed an NDA (or any one, for that matter) needs to be able to design new systems without regurgitating verbatim parts of an old one under NDA.
So it's really not a big ask to navigate their own NDA and come up with something interesting to talk about at an interview.
I’m going to be way more strict about confidentiality with an interviewer I don’t know at all than with a trusted colleague. For all I know you get beers every week with the CTO of my current employer’s largest competitor.
Besides which there’s a way more interesting variation on the question, which is “how would you design our product?” Got that one in my last big tech interview and quite enjoyed it.
It’s not a handicap. We’re not asking people to diagram their current systems. We don’t advocate for people to do that. You could just walk us through an HTTPS request and it would work, if you understood HTTPS requests well enough.
So it's really not a big ask to navigate their own NDA and come up with something interesting to talk about at an interview.