[Neil Jones]

I doubt anyone would think of Dad's Army or Porridge as 21st century sitcoms. The Porridge remake flopped and vanished without a trace (god knows why the BBC chose that one out of the 4 pilots rather than Goodnight Sweetheart), and Dad's Army was just a remake of the three lost episodes for Gold, not a full on reboot/remake, wouldn't be suprised if most people didn't even realise it had been made.

Just to jump in here, Gold have picked/remade animated versions of the three missing Dad's Army episodes, and they're shoving them out under the same presentation banner as the 2018 recreations. Not sure if these are the same ones that were on the BBC store:
www.comedy.co.uk 

Not that this leads any credence to the original point about 21st century sitcoms, of which Dad's Army is not one, though it is still on air today so you can tenaciously argue its only 21st century by the fact its never been off the air since it was made.