I am fairly new to collecting back after having sold my entire collection about 6 years ago. What really happened to D4C and why the entire database is gone? Of course, I remember a lot of familiar names from the old days posting here.
It's been a while now - I could not even remember what D4C stood for! 😆 Die Cast Car Collectors Club, I think.  The discussion forum software was out of date so they had to update to new stuff. Still under https://diecast.org/ . Most of the usernames got ported over but all the old content could not. Others will have more details about it... The cost of running the site is still supported by https://www.legacydiecast.com/ so buy from them! Â
The "car list and review" database is available at the top of this page, but its user interface makes it close to impossible to use, doubt it is being updated; we are still waiting that the database is made more user friendly. The forum archives are gone, unfortunately. The forums are working well so far (except that some people never resurfaced), and lately the auctions were resumed (don't have personal experience yet).