They're relatively common in SF, and increasingly common in LA for 4-5 unit apartment->ownership conversions. While they're weird for owners, they're attractive for small-time developers. Conversion to actual condo ownership is subject to regulation, and pretty tightly controlled by the city, but conversion to TiC requires basically no approval for builds with few (~5) units.

They're pretty attractively priced too, since they can be done so quickly, and because owning a 1/4 undivided share of 4 apartments, with exclusive use of one apartment guaranteed only by the strength of whatever contract a small-time developer can get drawn up requires some ... convincing