I can't tell you anything about having a semi deep socket, but I can tell you a story about NEEDING a semi deep socket and not having one. I was at work, and had to replace the driver's seat on a Lincoln town car. The seat in question was being replaced because the track was bad and would not move forward or aft. Since the seat was all the way back, the front bolts were a snap. The left rear bolt was a bit of a pain, but I got it out of there. The right rear was an 18mm nut on a mid length stud, and there was a metal tab directly in behind said stud blocking access from a wrench. A shallow would not reach, and a deep was so long that there was no room above the nut to put a ratchet on, due to the seat being in the way.
I searched high and low throughout the shop for a semi deep socket, only to find to my dismay that there are no semi deep sockets in the shop. Finally I asked my boss if he knew of any semi deeps, and he asked to see the bolt. He ended up getting it off with a combination wrench held at an angle and a little bit of time. I'm glad he is the man, because my plan b involved cutting the seat track apart with an air hammer and chisel.