The very last drawing by Herge is on wikpedia in the artical about Alph-art.
As has already been said in this thread, there’s nothing but circumstantial evidence for Alph-Art
being the last thing he drew - never mind that Wikipedia just isn’t authoritative enough a source to determine what came when.
It’s just one of the things he did towards the end of his life, but he wasn’t sitting at his desk with his pen in his hand, drawing Alph-Art
when he died.
From reading The Art of Hergé, Vol. 3
, it seems that Alph-Art
wasn’t even his last major project. The latest dates for work on that book apper to be in 1979 (although again he may have doodled, made notes or worked on pages after that).
However he personally designed and drew layouts for the Stockel underground station friezes - and those were worked on between 1980 and 1983 (thus after the Alph-Art
material, which he had set aside).
If you add to this that he was still involved in work on his annual greetings card, as well as autographing and sketching Tintin and Snowy on items for friends and admirers, and whatever other sketching he did, and it just isn’t possible on this evidence to say what the last thing he drew was.