I learned about six months ago from a status update posted by David Stubbs on Facebook that he was in poor health, and his non-involvement with the recent Barbican concert seemed to underline this - so not entirely unexpected, but very sad nonetheless.
Aside from his bass playing, he may have been the most important member of Can as far as the studio recordings were concerned - given that it was his task to take the endless reels of improvised recordings they made at Inner Space and edit them into releasable form.