Brett Powell has a process to find Power BI V1 workspace owners:
As most readers of this blog likely know, there are two very different kinds of workspaces in Power BI – V1 or ‘classic’ workspaces which are tied to Office 365 groups and V2 or ‘modern’ workspaces which are not. V2 workspaces have many advantages beyond their independence from Office 365 which you can read about elsewhere but for a bit of context you can read the GA announcement of V2 workspaces from back in April.
Since upgrading to V2 workspaces has been a manual process thus far, most Power BI tenants contain a mix of V1 and V2 workspaces. You may also have read the recent announcement of a new feature in the Power BI service available to workspace admins to upgrade their V1 workspaces. This blog post is all about identifying these V1 workspaces and their admins.
Click through for the process, as well as Brett’s recommendation regarding migration to V2 right now.