Yes the sphincter muscle is removed during a prostatectomy.
For the surgeon to be able to perform the prostatectomy the sphincter muscle much be removed otherwise they cannot remove the prostate.
In men there are two sphincters that keep you continent and allow you to hold your urine in and when those sphincter muscles are removed it can lead to incontinence.
So after having a prostate removed you may need to wear adult diapers to stop the incontinence from wetting your pants.
My father had his prostate removed and suffered incontinence afterwards and so he decided to wear adult diapers all the time now to prevent his pants from getting wet.
Sometimes he can still make it to the bathroom in time and so he just unfastens the diaper tapes and then pees and then tapes the diaper back on.
But he wears the adult disposable diapers just in case he can't make it to the bathroom and nobody can see if he has the diaper on.
Some people overcome the incontinence after a year or so of the prostate surgery but some people continue to have the urinary incontinence.