Have heat pumps improved much in the last 30 years, wrt to heating a
home in Baltimore?

I was under the impresssion that the limiting factor is often the
outside temperature, and if it's not over 45 or 40 or 35 or 30 (I forget
which), that it's hard to extract enough heat to keep a place warm.

And that since it's colder than that a lot here, the supplemental
electric heat will go on and that's much more expensive.

The salesman today, called by them a saleman, tells me that things have
improved a lot in 30 years and it can heat the house. But I don't see
how you can change the laws of physics.