Yes, for C+ fares they don't have any S bucket open in either direction (which is the cheaper Comfort+ bucket compared to W bucket class). But it seems fairly common to see routes where DL never has S bucket open on flights and only has the higher priced W bucket availability. They may or may not ever open S bucket on these flights at some point in future. But with C+ fares, there is also dependence on the coach class bucket inventory due to the dual inventory checking. The W/S buckets plus the coach bucket inventory availability act as a tuple to determine which specific C+ fares are actually bookable. If they were to open K or lower inventory on LAX-BOS-LIS flights, that would also lead to lower C+ fares being available for booking than current Q basis C+ fares.

The lowest possible C+ fares on a route would be an S booking class C+ fare with a V fare basis (an S+V fare tuple). Although with a number of routes, they don't always have fares as low as V class available for booking. It's not uncommon that fares only go as low as T or X class on certain routes (especially during heavy demand seasons).