It was added because too many people started entering by singing to karaoke tracks and I don't want to curate a karaoke contest. The focus of Open Mic has always been to bring a community of musicians together.
We do make exceptions if you sing really well and have a command of your voice like an instrument. There are also other contests on Steemit that focus on karaoke.
I was really thinking of entering acapella. I can appreciate your position regarding the karaoke-ness of it though.
I may enter in a coming week without accompaniment and if that means I'm disqualified then, so be it =) at least I got myself out there.
There are exceptions so give it a go. Also, there are other contests on Steemit that except Karaoke entries like Pitch Perfect.
You could also find a musician to play with.
Here at Open Mic we want to create a group for musicians who play musical instruments.