Jonathan Blow's talk 'Preventing the Collapse of Civilization' goes into detail about how technology can regress, with the Mechanism being one example. This kind of thing is far more common than we think, most would be surprised to learn that Ancient Greece had writing for about 600 years before forgetting it. There was no writing in Greece for over 400 years, until they adopted the Phoenician alphabet around 730 BC.…
Not sure if it is a myth, but hadn't we forgotten to build Saturn V engines?
The Space X Merlins have a much better thrust-to-weight ratio in the same gas generator cycle class, the Russian RD-180 powering the Atlas V is using oxygen rich staged combustion and is much more efficient.
And the current trend seems to be clearly liquid methane and liquid oxygen, covered by the phenomenal Raptor engine from Space X, the BE-4 from Blue origin and many smaller ones.
So hardly any regression on the chemical rocket engine front - pretty much the opposite, thankfully!
On the nuclear thermal rocket front on the other hand - yeah, we really did regress there. :P From almost flight ready NERVA examples in the 60s/70s to basically nothing even remotely flight ready today...