What's confusing about it? Is the confusion that there are 2 unicode characters (U+00A5 and U+FFE5) for the same thing?

The original yen mark for Japanese computer is "\" U+005C, that is rendered same as U+00A5.

U+005C is REVERSE SOLIDUS, it always has been.

Yes in theory, but in practice some Japanese fonts use the yen sign glyph for U+005C, the most common ones being those that come with Windows.

I wish it was before Unicode.

There is no "U+" before Unicode, what would that even mean? Unicode before Unicode?

0x5C in ascii compatible Japanese charsets

Possibly a security hole, similar to recent thread on BIDI and related Unicode complexity.