When you hear someone say that anything is the greatest thing “since sliced bread,” they mean since 1928
When the sliced bread was first sold, it was advertised as “the greatest forward step in the baking industry since bread was wrapped”.
So, if something is amazing, you can say that it’s the best thing since sliced bread. But if something is really amazing, you should probably say it’s the best thing since bread was wrapped.
Sliced bread was, temporarily, banned in 1943 due to World War II.
The toaster, in case you’re interested, pre-dates sliced bread with the first electric toaster being introduced in 1893. The pop-up toaster, however, was introduced at around the same time as sliced bread. Kids had to wait an another 36 years, until 1964, for Kellogg’s Pop-Tarts.