The reason is that in 9.0 we've prepared a new API for Runner requests (as part of new v4 version of API) and Runner 9.0 is using only this version.
Version v1 of the CI API is still supported by Runner 1.10.X and 1.11.X. On GitLab's side it will be supported until August 2017 and until then we will also support Runner 1.11.X.
The reason is that in 9.0 we've prepared a new API for Runner requests (as part of new v4 version of API) and Runner 9.0 is using only this version.
Version v1 of the CI API is still supported by Runner 1.10.X and 1.11.X. On GitLab's side it will be supported until August 2017 and until then we will also support Runner 1.11.X.