We moved to C8s with a Sig sidearm.
I think AOPs might carry a shotgun for polar bear defence, but I haven't seen their MAST (Mobile Account Ships Tables).
We also ditched our MP5s. I was sad to see them go, I loved that weapon.
Shotguns are a very niche firearm for LE or MIL usage, as virtually anything they do can be better done by other systems.
Neither Slug nor BuckShot are optimal options, as rifle caliber carbines from 5.56mm to 7.62mm NATO provide much better options in terms of precision and penetration, with a common manual of arms.
Less Lethal is one of the areas where the 12ga shines, as so many LE entities have lots of them, and so they are a economical option.
Most of those LL aspects can be done in 40mm for the Military, but it is not a good idea to mix a Lethal System with LL rounds -- one of the reason most LE LL Shotguns are painted specific colors to ensure the user is aware it is a LL system - and no lethal ammo is inserted.
C/UAS, for 99.99% of shooters, a Semi-Auto Shotgun is going to be the better option, given that most users are not internationally ranked skeet shooters - and shotgun training in the mil is a low focus effort.
In the ideal would it would be a C8 like shotgun (Hey Colt Canada make it so) loading from a magazine or drum using the same manual of arms as the C8/C7 series.
While I like the MP-5, it wasn't the easiest system to adapt between the C8 and MP-5, as the charging handle was oddly placed (compared to the C8), the lack of a bolt hold open on empty magazine, and the awkwardness of adding accessories. Plus 9mm kind of faded out when one needed to start considering shots beyond 150m, or targets that may be lightly armored..