Hello TQ
1. In the 21st century, WHEN so many traditional values are disappearing, it is comforting to know that some still remain.
— This is fine.
2. In the 21st century, so many traditional values are disappearing, it is comforting to know that some still remain.
— This is ok, but sounds a little breathless to me. It means:
2a. In the 21st century, so many traditional values are disappearing that it is comforting to know that some still remain.
3. In the 21st century, IN WHICH so many traditional values are disappearing, it is comforting to know that some still remain.
— It's a little unusual to say "to disappear in something", in the context of "values". "..during which..." would be better.
4. In the 21st century, THAT so many traditional values are disappearing, it is comforting to know that some still remain.
— No; "that" is the object of "disappear", but "disappear" is intransitive, in this context.
5.In the 21st century, WHICH so many traditional values are disappearing, it is comforting to know that some still remain.
— No; "which" is the object of "disappear", but "disappear" is intransitive, in this context.
MrP