> In what scenario would you ever need to use a sequence like <tab><space><tab> in indentation in your source code?

Writing a lisp in an editor that doesn't do boneheaded things with tabs? That's the only one I've personally run into but lack of imagination is hardly a good excuse to implement arbitrary restrictions.

> Let alone using esoteric Unicode whitespace characters for indentation.

How do you know what's esoteric in other countries? I certainly don't. I'm not an expert in linguistics but I'm sure that people everywhere in the world write computer programs at this point.

> I think it is perfectly reasonable ...

Without any concrete justification? Why would entirely artificial restrictions ever be seen as reasonable?