Vim/VI -- \ n can only be replaced or deleted. \ r can only be used for insertion or replacement.

% S/$/\ r/g
% S/\ N // G

In Linux, "\ n" indicates a line break, and "\ r" indicates a carriage return.

% S .... the/g search replacement format can only ensure that you are replaced multiple times in a row, but once you insert a row terminator (\ n), this row will be aborted, the current row is no longer replaced, so you obviously cannot replace it with characters like \ n, which will cause the current row to not be replaced.

As for how you can use \ n as the search pattern, this is obviously allowed.

"\ N" is used in the mode, but not in the replaced content.
Use "\ n" instead of "\ r ". The reason is caused by history. For details, see ": Help NL-used-for-NUL ".