James Malislewski talks about solitaire adventures here. One of the oldest if not the oldest soltaire for D&D was Judges Guild Survival of Fittest.
In my neck of the Woods (Northwest PA) it was respected enough that many DM would accept a higher level character that went through it. Of course you had to have a reputation for not being a cheater.
SOMA EP16: Queen of Humanity
3 hours ago