The HB has part of CA 91 west of I-110, to Vermont Avenue. The
Streets and Highways Code has 91's west end at Vermont Avenue.
West of Vermont Avenue, the Code says that former 91 is not a state highway, nor eligible to be added back to the state highway system. So any 91 signage west of Vermont Ave. seems to be just remnant signage, with no obligation for the cities that now control those segments to maintain route 91 signage. This differs from some relinquishments (usually in the middle of a route) I've kept in the HB for now, where local governments are required to maintain continuation signage to tie together the route segments still state-maintained, an obligation often but not always completely ignored.
How to handle relinquishments is something I'm still noodling over, including consistency with systems in other states (like Florida State Highways, where it seems all relinquished segments are omitted from the draft HB, even if that chops up a route into multiple disconnected pieces). But my tentative approach to California includes at least treating end-of-route relinquishments as truncations, which is what happened to CA 91 west of Vermont Ave.