When I joined my gamedev studio I had colleagues asking me why I had cash, and many of them didn’t even recognise what it looked like (there was a switchover of the notes a year or two prior).
There was an insinuation that I would use it for drugs. So, I suspect that the parent is right here.
Add.: another poster suggested that someone had a bit of a laugh with you by saying it, which is also entirely possible. Basic joke.
I believe them. Cash machines dispense(d) them, but usually only if you withdraw 1000DKK or more in a single transaction. That's unusual for people that rarely use cash.
I had them as I was following the official advice to keep some cash around in case the banking system is hacked etc, but I spent them and withdrew 100DKK notes when the advice was updated to point out that if everyone had only 500DKKs at home it wasn't very flexible.
I am slightly ashamed to realise that my banking app shows I haven't made a cash withdrawal in Denmark for more than 12 months — the second part of the official advice is to use cash occasionally to keep that system working.
See also: https://www.bbc.com/news/business-48993008
which as well as explaining the background to the note and debates over whether it should be taken out of circulation also points out the context the average British adult was making about 20 cash transactions a month as of 2018 (none of them involving £50 notes)
A friend's dad showed me one when I was at school - that's it. He seemed amused I hadn't seen one before, then after making a minor show of it, as if it was some precious, rare item, said he'd never previously seen one either. They've been uncommon my whole life, and apparently could/can be difficult to actually use, shop assistants being unfamiliar with them and not confident in their legitimacy.
I expect today most people would use bank transfers for the sort of sum where the sheer number of notes would make a £50 one useful.
No insight provided nor sources.
Just an opinion, a strong assertion, confidently stated with a tone of superiority.