Alright, let's start with the easiest example then. Green gas giants. With a few exceptions (when there was only a tiny blob of green), they are readily apparent from a glance at the system map. However, since they don't show up on the FSS barcode, the only way you could find one is to scan every single gas giant you come across.
So, how rare are they? Currently, there are 19 known. (They have separate entries on the Codex.) For reference, just on EDSM there are about 127 million bodies stored.
The numbers are available, you can take a look at them. But no, in a nutshell, exploration activity is either at or slightly below what used to be the norm. However, during DW2, activity was actually doubled, and once it ended (the end stretching from April to June), those elevated numbers dropped like a rock, and have been somewhere around the same as pre-Chapter Four these past three months.
Let's not forget that this is after the developers have showered explorers with credits and tags.