This was first done by SIDN I believe, the company that orchestrates the .nl domains. They however, do not implement most of the spec.

When searching their online whois[1], they'll tell you if a domain is for sale and link to the for sale page. They don't use any of the info you put in the DNS though. The URL comes from their internal system. You need to register it separately. They also don't use the price, txt or anything else you put in the DNS.

All they use is the "fcod", to look up the url in their own system. They had a different syntax for this before, but this functionality has existed for some years.

[1] https://www.sidn.nl/whois