Well 1.8 is generally the best version for parkour as there is the most variety.
As for why we can't facilitate for different versions, well that's because different versions have different physics, and hitboxes.
-Ladders are 1 pixel bigger in 1.9+
-Short panes no longer exist in (idk what version)
-In 1.9+ the minimum speed threshold changes from 0.005 to 0.003. Essentially a minimum speed threshold signifies how slow you can go before your speed is rounded to 0. This makes a big difference in more advanced parkour.
-As a result of the min. speed changes, you can jump 1.252 blocks high in 1.9+, as opposed to 1.249 (very significant as there are many blocks 0.75 and 0.25 high)
-In 1.9+ Single panes turn from x shaped into 1 vertical column
-In 1.13+ The top of an anvil becomes 1 2 pixels thinner and the base becomes standable.
-Blip ups add 0.121 to your height in 1.9+ instead of 0.104
-You cannot quad blip in 1.9+, while you can in 1.8 you can
-Blip ups are patched in 1.14+
-Z facing no longer exists in 1.14+, x facing blockage is reduced
-Your shift height goes below 1.5 in 1.14+
-You can sneak on the edge of blocks less than 1 high that are not steppable, this was not the case in 1.8. This makes blocks like the brewing stand much easier to use.
-Lily pads are 1 pixel higher in 1.9+, making them higher than carpets by half a pixel.