SSMS 17.9 provides support for almost all feature areas on SQL Server 2008 through the latest SQL Server 2017, which is now generally available.
In addition to enhancements and bug fixes, SSMS 17.9 comes with several new features:
- ShowPlan improvements
- Azure SQL support for vCore SKUs
- Bug Fixes
View the Release Notes for more information.
It looks like the big push for this release was bug fixes, and there are quite a few of them.