History mode is currently in place for the CAMPAIGN, EMAIL, LIST, and LIST_SUBSCRIBER tables.

I would like to request this feature to be added for TRIGGERED_SEND 

In the current state, this only contains a reference to the EMAIL that is used now. There is no ability to look back in time/an audit trail of which EMAILs have been used previously.

This is needed as while the TRIGGERED_SEND contains say an abandoned cart master reference, the email_id is updated over time as different EMAILs are utilized. That means there is no way to tie an EVENT through to the actual EMAIL that was used over time, which is needed for analytical purposes.