I just thought that it would be an interesting statistic to know which openings players like Carlsen and Fischer lost most against. Are there any websites that give these statistics?
Opening tree happens to be the website you are looking for. Using this, you can use a database of games or someone's Lichess or chess.com username to get detail about their games. It shows frequency, last played, best win, worst loss, win-loss stats and performance. It also have have fatabases of famous players in it. Checking Anand's games as black, after playing
1.e4 2.c5 3.Nf3 his worst performance is in Sicilian Defense: French Variation (elo performance 2644) with score 107/173 and worst loss was Ljubojevic - Anand 1998.
Overall, it's a nice free website.
lichess.org has also this kind of tool for open profiles. I guess this tool is used or can be used for external chess websites. Probably, someone can find the open-source code of chess-insights tool in lichess-github repo. Read more about chess-insights here