Channels with limited retention

Description and Why This is Needed

Currently, there are only two and a half choices regarding to message retention: persistent, persistent with hidden by channel visibility, and ephemeral. Some chats are rapid-fire and we know that issuing many message deletes at once are not really compatible with how Discord back-end works (https://blog.discord.com/how-discord-stores-billions-of-messages-7fa6ec7ee4c7). Therefore, I propose a new feature, channels with limited retention.

Alternatives Considered

Bots regularly deleting messages with singular delete requests (bulk deletes cannot be scoped further back than two weeks, so it cannot be used for this purpose), but this worsens performance in Discord's server side.

Additional Details

At each channel creation, a retention period should be able to set by channel creator. This retention period can be altered afterwards, but a persistent channel cannot be converted to limited retention or vice versa. The channel retention period can only be altered by those:

• guild owner (both upwards and downwards)
• user that has power to delete the channel (both upwards and downwards)
• user that created the channel, regardless of whether that user still retains the power to create a channel (downwards only, and only if said channel's retention settings has never been changed by the above two)

Similarly with guilds. Guild retention settings can only be altered by guild owner, with the same constraint that a persistent guild cannot be converted to limited retention or vice versa, and can be overridden by shorter retention periods in particular channels. Retention changes should be applied going forward, to prevent complicating the back-end. Retention setting should only affect events that appear in the channel timeline, and not the metadata or permissions. To simplify things, those retention periods should apply from the time the message is sent.