There used to be a really cool tuner made by Sequerra which I saw in
the 1970s which had this amazing feature on a built-in scope. None of
the other tuners made at that time which had scopes in them seemed to
have that function. (I actually bought a Marantz ST600, though for the
AMAZING sound, not the scope.)

Does anyone know why this technology died a death?

It adds to the tuner's cost without providing a feature most users need.


Surely this could be useful now, as well as being useful for broadband
signal detection, showing signal strength as well as how stable the
signal is?

You can buy an inexpensive WiFi detector at RatShack and elsewhere.


Or has the concept of having a scope in a tuner died a death for some
reason?

See above.

FM is no longer a significant program source for many listeners, especially
with the decline in classical and jazz stations.