Public room directory response should allow for extra fields

[Half-Shot]

The public room directory response currently allows a static set of keys for each room entry. It would be good to be able to extend this field-set optionally without having to go off spec, in the same way we allow events to be of any type and contain any content.

For instance, information from MSC2346 could be included to announce bridges in public rooms. Being able to include arbitrary state would be useful. While this information would be impossible to verify, it's no worse than already trusting the avatar/topic/name.

[turt2live]

any JSON response can be extended with a namespaced key - what is the ask here?

[bkil]

I think we might want to create a meta-issue or label to deal with improving the public room directory and explorer. I found the following other issues so far:

• Metadata for rooms to flag as NSFW #210
• Extended information in rooms #314
• Add a new state event to describe the room content #782
• Allow the room directory to order by relevance rather than population #739
• MSC3266: Room summary API matrix-spec-proposals#3266
• Room Directory Search should use Text Search on Postgres synapse#5747
• Room directory search does not find hyphenated words synapse#9303
• Trending Rooms / Other ways to discover rooms element-hq/element-meta#1480

What I would find useful to see next to each result at the Explore Rooms tab:

• Whether the new joiner could read back history to access previously built up knowledge base
• Whether the new joiner would have permission to speak
• Whether the room has live moderators: how many members have PL good enough for moderating, from these, what is the date of the most recent read receipt or sent event
• What is the estimated event rate within say the last week - I would most of the times rank results based on this one instead of membership count
• What is the active membership count within the room (those who have sent any event or read receipt within the last month)
• What kind of bridges, bots and other integrations are hooked up to the room, even if only marked by icons
• A URL to the privacy policy (logging, analytics), terms of service and code of conduct of the room that one needs to accept before speaking
• Whether it is a space or a non-space room
• Spaces that the room is part of (and/or which the room would like to advertise by hand)
• Languages spoken within the room (the primary one and others as well)

It would be worth it to provide a search filter for some of these as well.

[turt2live]

In an ideal world, the room directory just disappears and becomes room peeking and/or a space. That is significantly harder than defining some stripped_state which should be returned.