I have noticed all the official DLC tracks now need a lead, rhythm and bass part. Is that the new standard now?

It's probably just because the bands have 2 guitarists and one bassist. It's not really a necessity as you can find official track that doesn't have both lead and rhythm but it's pretty rare.

I think it's still "up to the creator", but they may have changed their suggested workflow. Past official DLCs would sometimes have one of the 3 missing (resulting in an X if you were on the missing lead/rhythm/bass path), but recently it seems they've been duplicating the guitar for both paths if there's only 1 guitarist & no overdubs (the Primus pack does this, making lead & rhythm identical), and in at least one case even writing a bass line for a song that didn't have bass in it originally (can't remember which song, but I remember them mentioning it).

Thank you!