I am looking for a good chess website about retrograde analysis problems, but after a few searches, I haven't found anything. Are there any websites that you can recommend?
I would recommmend The Retrograde Analysis Corner It is a top notch and well known site that explains castling and en passant conventions, what exactly it is all about, and how to solve such problems among it's many, many helpful things.
Do not miss Rund um die Retroanalyse, a blog by Thomas brand, in German language. It features each week a retro problem, with explanations and the problem solution.
There are literally thousands of retro problems at https://pdb.dieschwalbe.de, accessible through the easy-to-use query interface.
There are already discussions for many of the most interesting ones, and if you have any questions or comments, you can just add your own.