SQL Server Reporting Services 2017: Multiple Instances
From my research, this is currently not implemented in SSRS 2017. I was not able to find any information in an official Deprecated Features document.


Comment from user 'Matt'  
Comment from user 'Sai_Kumar'  
Twitter-Post from Christopher Finlan, Senior Program Manager, Power BI Report Server &amp; SQL Server Reporting Services at Microsoft.


I found two votes to bring this functionality back to the product:


Support Power BI Report Server multiple instances on a single server  
Restore multiple instance support for Reporting services in SQL 2017


Update (May 2019): The feature will not be reimplemented


  AdminChris Finlan (Program Manager, Microsoft Power BI) replied:
  
  Hi all, We aren’t planning to re-implement multi-instance support as
  we had prior to SSRS 2017. We recommend using virtualization as an
  alternative, and we’re also looking at possibly adding container
  support in the future.