That is completely false.
For example I have never undertaken a mission (or salvaged a combat ship) that gave Exquisite Focus Crystals yet I have 21 in my inventory and I have used some in Engineering. The same is true for other things in raw, manufactured and encoded.
To check (and prove to myself that I was not imagining things) I just did a quick trade at the station I am docked at for an encoded material that I have never found, received or traded for before - so starting from 0 Divergent Scan Data, I traded 6 AEA to give me 1 DSD. Like this:
View attachment 422858
I could do the same with manufactured and raw materials ('cos I am new) but I can't be bottomed.