[zimpenfish]

RFC1738 is obsolete. RFC1866, referenced in that SO answer, is also obsolete.

But none of that matters, particularly, because lastfm are consuming the URLs, they know full well what "+" means in a URL submitted to their servers, and also they've been around for decades (well before the whole "use %20 instead") - why on earth would they change up their URL scheme when it's working fine?