Acceptable to who? Acceptable for what?
Lowest BB I ever owned was a 1967 Falcon M90. When Falcon was still totally handbuilt. Depending on tires glued BB height, ground to center, was 9-3/4” to 9-7/8”. This was before road tubulars narrower than 25 existed. It was a 23-1/2” frame and arrived with 165mm cranks. Wide Campy pedals of course and they scraped constantly. Which is not a problem unless you are determined to make it a problem. If scraping a pedal is going to throw you into a complete panic don’t ride a bike like that.
By 1967 that frame was a throwback. Earlier British bikes it would have been frequent.
Lowest I ever saw was the frame Ron Boi built for himself and raced Category I in early 70s. Bottom bracket drop was 101mm, coming to a height of about 9.3”. His review of his own bike was that it was completely impossible to pedal into a corner but he still came out of the corner quicker than anyone else. He got better draft off other riders and gave less draft to his opponents. He rode with 170s. He never convinced anyone else to try it in his next 45years of frame building.
Most builders will decline to go that low. You won’t be able to argue with them about the point. You might find someone who would do it. Most are simply slaves to fashion. Fashion trumps function every time.