I think as you can see from the answers you’ve received that this is a controversial issue. There is much research on both sides of the issue (Deep Knee squats are safe vs. Deep knee squats are not safe). As LaRue points out, I would review the research and make a qualified decision for yourself, vs. youtube videos.
As for the client that you mentioned, an older than 40 just getting back to exercising. I’d be concerned about ROM and knee tracking with a deep knee squat. I’d start with other foundational exercises with a person just getting back to exercise or new to exercise no matter the age. For me personally, I do not consider a deep knee squat a foundational exercise.