Interestingly, Decker said "what they used to call a black hole". Did people stop using that terminology for the phenomenon later on?
IIRC, black holes had only recently been discovered when TMP was being written and filmed. Back then, people thought a black hole was something that a ship could fall into, pass through, and emerge on the other side....
Not exactly. What we would call a black hole was theorized long before the original Star Trek, and the actual term "black hole" took hold in astrophysicists in the 1960s. Sure, Einstein Rosen bridges got popularized in science fiction, but by the time of TMP there was no lack of understanding about what black holes were and that they were most likely a one-way trip.