You can’t limit the clients which connect; it’s an open protocol. If the data being stored in the rooms can’t be represented as text messages then you will see empty rooms - but the point of MSC1767 is to try to ensure that you do see a relatively meaningful representation of custom structured data even in a plaintext Matrix client. For instance a video game could have a bunch of KV state events in the room representing game objects. With MSC1767 they could have simple fallback representations visible in a generic Matrix client (eg “chair” or “door”) or more sophisticated HTML representations (eg an html table showing you that object’s name and spatial coordinates and velocities). Or a more sophisticated but generic matrix client might be able to map out generic 3D or 2D scenes without actually understanding the game specifics. Basically, think of the generic Matrix clients as acting as a debugger console which at least gives you some way to interact with a room - and lets generic bots/bridges interact for the simpler generic use cases like IM and VoIP.
