There are already many organizations that have disabled O365 group creation for users as they want to provision this for users instead (e.g. Microsoft Teams). Haven't seen any documentation from Microsoft advising against this. They are actually highlighting this option, as you can see here. This already disables the connected group feature in Yammer. AFAIK, stand-alone Yammer groups are not going to change as long as Yammer freemium isn't removed as an option. You're just getting less options (i.e. no live streaming) and you can always convert them later. Most if not all of the innovation will be restricted to connected groups. This is why I do recommend having a process for users to request a connected Yammer group even if you have a provisioning solution in place.