The main Power BI blog is the best one to follow https://powerbi.microsoft.com/en-us/blog/ however PBIRS does not really have it's own road map. It really just follows the cloud version and each quarter they merge in changes from the cloud version that make sense in an on-prem environment.
Typically things like performance improvements, new DAX functions and 3rd party visual API updates all come through. It's things that require cloud infrastructure like some of the AI functionality and many 3rd party connectors that may not make it to PBIRS as the infrastructure is just not available on-prem to support them.
There are a few features that are specific to PBIRS, the new v2 portal experience is one example, but they tend to be announced on the Power BI blog when they are available.
