As Dean says, there’s nothing in-game.
As well as Facebook groups, there are various Discord communities where post-race chat happens; but those tend to be team-specific. (e.g. I’m in Discords for the DIRT and Backpedal communities)
It’s a subject that has come up before (e.g. Post Race Parking Lot Chat), and there have been feature requests in the past (e.g. Post Race Warm-down from 2017)
About a year ago, @xflintx posted a post-race discussion template to the ZwiftPower Community - Zwift Forums section:
It hasn’t got a lot of traction (Search results for 'post-race #racing:zwiftpower-community' - Zwift Forums) but maybe there are ways it could be promoted more (e.g. a link to the forum on the Zwift Community event results page)
In fact, I just noticed @shooj posted about just that in the thread.