The sad thing is I knew this. From a reader:

Every true Transformers fan knows that Optimus Prime
died in 1986’s “Transformers: The Movie.” It included
the climactic battle of life and death between Optimus
Prime and Megatron at the START of the movie, in which
Prime was bested and died…before passing on “The
Matrix” to his successors. It was a pivotal emotional
point for many guys today aged 23-32 I suspect, as it
showed the mortality of one of our superheroes.
The movie was notable for many things, including lots
of robot deaths, profanity (one of the human
characters says “Oh s**t”), and Orson Welles as
Unicron, the planet-sized hulking mass of destruction,
which mirrored his own life at that point in history.
Also, showed us that battles have consequences, unlike
the afternoon cartoons of GI Joe and the Transformers
series where there would be huge fights with ammo and
lasers and bombs, but only a few banged up good guys
and bad guys at the end of it all (remember how the
pilots in GI Joe would always eject at the last
possible minute, escaping death?)
R.I.P. Optimus Prime.
However, as most 1980s pop culture icons, he lives on
and has been reborn in re-imaginings of the series.
To wit, major motion picture projects are up and
running on the GI Joe, He-Man and the Masters of the
Universe, and TRANSFORMERS series, the last one being
produced by none other than Steven Speilburg.