I have a counterpoint regarding removing dead sites. While I agree that the new system disadvantages newbies, removing all dead sites would disadvantage older players. Now, I myself am a "newbie" and I think I am unfortunately disadvantaged by the loss of some old sites (i.e. ThisIsLegal, HaxTor, and CSTutoringCenter all look like I might have been able to make progress on them, but that's of course just speculation). The reason I say I'm a "newbie" is that pretty much none of the sites I have scores on are down. Out of all the sites I have linked, 3 are down: LOST-Chall and Yashira, on which I had pretty low scores, and DefendTheWeb, on which I had completed every challenge.
Now, if we want a preview of what a ranking without dead sites would look like, we could look at the Active Sites ranking. On that ranking, I am number 15, right behind Caesum. However, had Defend the Web NOT been taken down, I would actually be ahead of Caesum, because the difference in our scores on Defend the Web was large enough that it would make up for the gap between us on the other active sites. This happens because in effect when you remove a dead site, that is equivalent to assuming that every newbie could complete every challenge on it. Is that not an unfair assumption? In fact, the fact that I can even come close to passing Caesum on active sites demonstrates that it is an unfair ranking, because I am not better than Caesum! If you look at just about every site that was popular when Caesum was active (bright-shadows, rankk, Revolution Elite, HTS, etc.), Caesum beats me by a long shot. So obviously, Caesum should have a way higher score than I do. So why are we so close on active sites? Because many of Caesum's difficult achievements are on dead sites, while Caesum didn't have access to a lot of new sites because Caesum's last activity was in 2019! Since then, many sites have joined that allowed me to rank up quite a lot of points (i.e. pwn.college, HTW, etc.) that Caesum never had access to!
The point is, while the current ranking is unfair to new players, removing all dead sites would be unfair to old players. We need to seek a different solution.
I want to stress that, if anything, I should be biased in favour of adopting active sites. It would move me from #39 to #15. It would make further progress for me much easier. However, while I do believe that a change needs to happen, removing all dead sites is pretty clearly unfair. I don't really know what the solution to this problem is, my best idea so far is to subtract the average score on a dead site from the user's score, so that what is measured is how unusual their performance was, rather than how many points they got. I have not researched this idea however, so it is likely terrible.
Anyways, sorry for the long message, just wanted to share my thoughts.