I guess making a server that is both compatible with 1.8.x and 1.9.x is a little difficult, mainly since there are a lot of clientside mechanics added in 1.9, such as attack damage that varies and attack speed that changes how much damage certain items do, and the whole 'Main hand' and 'Shield/building blocks slots' affects client side gameplay that cannot be changed with the magic of plugins.
Eventhough, I believe that 1.9 has a lot of cool new features and it fixes a lot of bugs (Take for example the painting pictures in the lobbies with custom images, they will no longer dissapear if their hitbox is offscreen, because 1.9 treats entities all new). Also, the new attack speed mechanic might prevent some hacks, such as killaura or range, since faster swinging will not let you do more damage. (But I highly doubt that the creators of hacked clients will not find a workaround for that :( )