It’s always been just KDE to me. In the past year or so, I’ve noticed people calling Plasma KDE or even just Plasma. What happened? Why do people insist on making the name longer? Why the word Plasma? What is Plasma referring to? And, are they trying to move away from the KDE name to just Plasma?
Answer: Plasma is the desktop environment. KDE is the community that makes Plasma and many other related software.
KDE is a community of developers.
KDE Plasma is the desktop environment made by the KDE community.
This has been the correct terminology since 2009, much longer than “the past year or so”, but a lot of people do still call KDE Plasma just “KDE” colloquially, so maybe you have just been noticing people using the correct terminology more recently.