I think it is not gonna be getting added because some jumps are possible and some don’t trough the changes in 1.9 think about the glass panes still a great idea I think most of the games would be great of getting on 1.8
I believe that this will neither be added, nor will it be necessary.
I understand your preference for the 1.8 versions of Minecraft, as I've been using these for the longest time as well, however trust me when I say that the other versions are completely fine as well! Especially for Parkour, you won't notice much of a difference. I transitioned to using 1.12.2 some time ago and barely felt a difference when playing Parkour, instead I felt like known glitches on transparent blocks occurred much less!
I don't think parkour will be added to the 1.8 server. Some blockjumps that are in 1.9>, are impossible/ easier/ harder in <1.8. Eg: glass panes. I don't think it's worth the time to adjust all of the parkour maps, just so it could be added to 1.8.
It'll probably not get added. As other people said, parkour maps with glass panes will have to be revamped. And you can just go in 1.9, it's not that much of a difference.
Fun fact: You jump higher in 1.9+, about 0,00081 blocks.