National Gallery thinks she's alive and a quick check doesn't bring up any death certificates for a likely Schuster in Scotland to December 2018.
She may of course be long gone (I've never heard of her!) but absence of proof so far.
Marsh was apparently rehearsing, felt faint and woke up in hospital. Tried to leave only to be told it was Intensive Care! Thankfully she made a swift recovery, but yes, she was been a lot quieter in general since.
Atkins is still acting - was in Doc Martin earlier this year - and looked in fine fettle in that thing with Dames Smith, Dench and Plowright last Christmas.
Obvious statement here, but I think all the Great British Acting Dames from that period of time (those 5 plus Sian Philips and Diana Rigg) will all make fine DL regulars one day.