Even then, are you sure it's "treated by the city as a public street"? On https://www.axisgis.com/CambridgeMA/?CamaIdName=[CamaFullNumber]&CamaIdValue=120-86 shows it as within private property. It has a name, but so does Cottage Row. It's in light gray, but so are private driveways. There's a gate (usually open) and an ambiguously placed "no trespassing" sign.

(I don't really think how the city treats it is the right question, but if it's an established "local mapping practice", then I guess this isn't the place for me to debate it.)