Well the coupler on 3 of mine are not, but the Bay with the electronics are.
However , I’d probably say that they are vented somewhat from the brake line that feeds the igniter wire from the sustainer.
Some of the kits that I build were designed with vented inter-stage couplers ( Estes Solar Flare (2401), Semroc OMEGA (KV-64)), and some of my own scratch builds have them. I'm especially inclined to do it when there's the rocket is gap staged. You want to vent the booster to prevent it from separating from the sustainer without igniting the upper stage (I've had a couple of rockets that had that problem (Estes Magnum Payloader (2032), Estes Warp II (2022))... The results were not pretty).
9 times out of 10 there's an altimeter in it, which requires proper venting. The other 1/10 is when I'm doing motor eject for the booster chute deployment and using the sustainer's motor for separation, assuming that drag separation doesn't occur first. Obviously, the sustainer ignition is being handled in the sustainer's AV bay.
Yes for av-bay/coupler [booster] section. Not the tapered transition section, for me anyway.
All electronics are always in coupler part of I/S and vents for altimeter are through booster airframe into coupler.
If the booster and the sustainer are the same diameter, I usually vent, to prevent over pressure when the 2nd stage lights. (Hasn't happened to me, but I have seen it happen) If the transition is tapered, the booster is a larger diameter than the sustainer, it will usually drag sep, so no need for vents. I think this is what you were asking about.
The only rocket I have multi staged was my Viking rockets up to 3 stages. I just taped the motors together with scotch tape. The boosters were all the same length as the engines, making it pretty simple and always very reliable. I recovered all the boosters, but lost a few sustainers that went pretty high using C-6 engines.