I will highly disagree with that. With different compatibilities, especially with major ones like 1.8 + 1.9, you will pretty much need to code everything so it works for 1.8, and not 1.9, otherwise people win 1.8 will not see the same things as people in 1.9, therefore giving a great disadvantage. Furthermore, little features like the pushing function in 1.9 only applies for people in 1.9, and not 1.9. Picture this: two people are doing lobby parkour, one in 1.8 and one in 1.9. They both jump to the same block. The player in 1.9 will get pushed off, whereas nothing happens with the play in 1.8. If you add up all of the differences and disadvantages from both version, having cross-compatibility will be unreliable, for both the future and the present.