I seem to have found the first answer. PLEX was made in 2008 but only obtained in game from ETCs until 2009 where CCP store begun selling PLEX, and 2014 where ETC was completely removed 3rd party sellers sold PLEX too.
I found a post stating it was available from the CCP store since 2009. But also that you could convert it already in 2008. The link I found to what seems to be the release message for PLEX just gives a server error. But I did find https://www.eveonline.com/news/view/the-etc-is-dead-long-live-the-plex-activation-code which explains a bit about ETC and the original partner sold PLEX packages.
I found ETC is Eve Time Code, but how did you get these? And how did they come into the game to be converted?
There was also this stuff called Aur, Aurum or something that was all converted into PLEX at one point, as far as I understood this was originally used for the clothes store. Very hard to search for Aur which it is commonly called in posts though, thanks to Arch Linux …