Analog really doesn't seem too interested in selling these. Those lead times ...
In an ADC market filled with boutique rip off prices these always stood out a bit too much.
Yes, it is an odd part, but I've heard from a few people familiar with the market for ADCs and if you know who you are talking to, you can get inexpensive Chinese parts with surprisingly decent performance that easily beat Western equivalent parts in terms of performance per buck. A great deal of that has been driven by the budget oscilloscope and test equipment market, as well as cheaper RF SDR and amateur radio kit. Digi-Key and the likes only tend to capture mainstream parts that are worth stocking.
Fundamentally there's not much that's too specialised about ADC design now - these designs are decades old and we have audio cards with 24-bit ADCs running at 192kHz ... this is sort of like the opposite end of the performance spectrum - it's a process problem, not a design problem.
The HMCAD1520 is available on Digi-Key, they have decent stock (~299 parts) and a 14 week lead time for more, which seems OK to me. I had no issue buying the HMCAD1511 when building the first prototypes, though I only bought two.
I'd imagine ADI only keep these parts and don't develop additional variants because they have existing customers that are happy with them from when they bought Hittite (the part actually comes from Arctic Silicon's "Blizzard" family of ADCs. They are/were a Norwegian firm that Hittite acquired before ADI acquired them.) But, it would be nice to see more lower cost parts.
My plan is to figure out a multiplexing arrangement where two ADC chips could be used to sample at 2.5GSa/s. I have already managed to get a HMCAD1511 stable at >1.2GSa/s. That would enable a realistic 2.5GSa/s oscilloscope (400ps sample period) with say 350MHz per channel B/W in 1 channel mode. I also suspect that the '1520 features might only be lasered out (or they may not be disabled at all!) on the '1511, as the two ADCs seem to use very similar cores/structures - though I am yet to confirm this.