Normally I would break down a television show this long with a episode by episode breakdown within each season. However, since this is a Sit-Com with very few deaths, I am going to encapsulate the particulars in a single entry.
It is tempting to include Professor Proton’s death which we learn about in season 7 episode 22, but it is heavily implied that his death happens outside the confines of the episode, so will not be counted.
In season 8 episode 15 Howard’s mom dies.
In season 11 episode 13 it is revealed through exposition from Sheldon’s mom to Sheldon that a Mr. Watkins has died. (This character is never referenced in any other episode).