Underwear Stalls still in Silom ??

[Patanawet]

From what I see the stalls are allowed on Silom itself only on certain days and not every day as before.
Patpong is a private street and can regulate itself I presume.

Not quite true.
Street markets are not permitted in Bangkok on Mondays (except, as you say, places like Patpong which are private property).
Furthermore, some streets like Silom do not permit markets any day. The vendors are wily and set up stalls on shop steps, courtyards etc.
So you will still find most things you are looking for on Silom (including underwear galore).