In almost 40 years I don't think I have EVER seen a plainclothes officer (detective or dignitary protection) carry an IWB holster on duty.
Lots of guys have experimented with IWB or AIWB for carry of off duty guns and some liked it and some did not. I have a Milt Sparks Summer Special rig that I got in 1990 or so that will fit my Sig 230 and the Walther PPK and various small Kahr and Glock pistols and I use it sometimes in the summer. With my build I find OWB to be more comfortable. I also have a variety of shoulder rigs I have accumulated over the years, many made by the original Famous Jackass Leather Company of Chicago (that later became Galco of Phoenix) and one by Mitch Rosen and one by Greg Kramer.
Over the years I have known a few guys who got special permission to wear shoulder rigs concealed in plainclothes. Usually they were older detectives with back problems.
When I first started there was a Detective from the Sheriff's Office who (briefly) used a Jackass Shoulder rig to carry two S&W 59s. (I think he did that because he thought it was neat and he was waging psychological warfare with the Chief Deputy at the time. He didn't do it for long)