Ok, let's clear up some confusion.
First: The guitar is a transposing instrument. It sounds one octave below written pitch.
Second: The guitar is a "C instrument". What this means is that the guitarist reads "C", they play "C", and we all hear "C". Regardless of the tuning of the individual strings, this fact still applies.
In music, with respect to strings, we don't call it a "transposed standard tuning". The fancy word here is scordatura which just basically means "alternate tuning". What you're talking about is merely scordatura, which is something that all string players deal with.
To be absolutely clear with respect to transposition: If you did have a guitar "in Bb", then the guitarist would read a "C", would play a "C", and we would hear a "Bb". Since this is obviously not the case for many styles of music, the guitar therefore is, the large majority of the time, a C-instrument.
However, especially in popular styles (rock & roll, blues, funk), utilizing a capo may cause the guitar to be treated as a transposing instrument (though fairly common, this practice is not a universal standard among musicians). In popular styles, the notation will inform the position of the capo, with the chord-notation indicating the shape of the chord relative to the capo and not the sounding pitch. For example, a 1st-position C-major chord on an open guitar will sound as a C-major chord, but a 1st-position "C-major" chord on a 3rd-fret capo'd guitar will sound as an Eb-major chord.
That said, it should also be noted that even capo notation only relates to chords and that any individual notes traditionally notated on the treble clef will not be transposed, despite the capo. Further, it should also be noted that in addition to transposition being a non-standard performance practice of guitarists, notation I described in the preceding paragraph (which includes numbered tablature) is also non-standard, with both performance practice and notation usually associated with the lay musician (not a concert-trained classical guitarist).
If anyone here (or elsewhere) knows of specific, traditionally notated "serious" concert music wherein both chords and individual notes are notated for a transposing guitar, please add the resource in the comments. Many thanks to Todd Wilcox for the suggestion.