Application Dependency/Limitations of Postgres Version Upgrade

I would like to know about application dependency of PostgreSQL version upgrade.

We have multiple servers with PG versions in 8.4, 9.2, 9.3, 9.4 and 9.5, that we want to upgrade to 9.6. We want to be sure that all applications will run smoothly after upgrade.

Is there any dependency or limitation of applications in 9.6 after upgrading from these versions ? OS platforms we are using are in Linux 5, 6.7. 6.8 and Windows 10, 12. 

I am happy to provide more information if need.

Regards,

Amitabh

PostgreSQL DBA