Paul, good essay. Responding to your comment on Sectional color codes, the TRSA boundary lines and associated characters are defined as “black” but in the printing process they are screened, for clarity, to about 80%, resulting in what looks like charcoal colored lines. Small detail, but the question came up several times through the years, and as a Flight Instructor at Palm Springs, I looked into this as I needed an explanation to flabbergast students.
KPSP is a part time Class D airport with a part time TRSA. The PSP TRSA was a consequence of a fatal accident. Uproar led to a radar facility that was subsequently upgraded to an ASR-9 then to an ASR-11 radar, now connected to SOCAL at Miramar, some 66 nm SW. No more Up/Down ATC at KPSP.
KPSP is very close to high terrain with an interesting mix of traffic, both fast and slow movers, compounded by traffic from two other airports, KUDD and KTRM. Thus, the PSP TRSA is in the books as somewhat of a small hub serving all three. It was a good thing when first implemented and I think that it continues to serve well. I instructed there for over twenty years, having enjoyed the wild ass rides due to winds and thermals. KPSP was one of my landings during my first solo cross-country flight back in 1967 and I was fairly impressed while going through the Banning Pass then on the approach to the landing. That Up/Down part remains.