Ineed, but unless I'm mistaken, Canon have not released user-downloadable firmware updates for any of their premium L lenses, whether they will in the future is another matter.
There's really not enough for a precedent. Only 5 lens firmware updates have come out, one for a $150 consumer lens and the other four for supertele lenses >$7000. But relatively speaking there aren't that many 300-600 MkII lenses out there, and there were even fewer at the time of the update (my 600 II came with the updated firmware, about a month after the advisory was issued). OTOH, there are a lot of 24-70/2.8L II lenses out there, representing a major burden on the service centers and a substantial outlay for shipping costs – I highly doubt they'd require sending the lens in.
As I said, whether they will in the future is another matter. But
they have not retrospectively made the firmware updates for the L lenses user-downloadable, for the handful who may have missed the advisory notice.