At least in central San Diego, the phantom streets seem to be areas where the city owns or owned the land and had an option of building a street there, but which never (yet) ended up being created. If you check many of them out, they tend to be alley ways or narrow vacant lots.

In a South Park micro mall recently, one store had on display an old map with no date, probably 1930ish, with a fascinating mixture of reality and hopes, like the real Rancho boundary along Boundary St., and the imaginary cluster of streets developing all the canyon bottoms by Hollywood Park.

Here is the explanation on their web site....

The data from The National Map Seamless Server is public domain. There are no restrictions on data downloaded We request that the following statement be used when citing, copying, or reprinting data:

"Data available from U.S. Geological Survey, EROS Data Center, Sioux Falls, SD."

A request is not a requirement. I won't be citing, copying, or reprinting
the photos, at most just creating a derivative work.
I think my notice here is sufficient and reasonable.