Does REST API support on-premises Report Server?

yes.

Chris Webb has a blog post

https://blog.crossjoin.co.uk/2017/10/15/exploring-the-new-ssrs-2017-api-in-power-bi/

I setup a connection to the Oct PowerBI RS 

using http://Server/Reports/api/v2.0

and connected to an odata data source

drill into Datasets and get the ID

They use in an odata conection with open type columns (from advanced) and put your id between the brackets.

Source = OData.Feed("http://server/Reports/api/v2.0/DataSets(06ffbf58-b11a-47d0-b17c-18ff44d26c68)/data", null, [MoreColumns=true]),

And expand your columns.

see for more details

https://powerbi.microsoft.com/en-us/documentation/reportserver-access-dataset-odata/