Until v14 it was possible to configure the messages of the channels as "Send messages by email" allowing to use the channel as a mailing list.
It was also possible until v15, when creating a Channel, to choose between being a Public or private Channel.
How to configure mailing lists and private channels in v16?
Odoo Answer:
...
Some changes were in fact implemented for Discuss starting from version 16. It is still possible to send a message in the form of an email to any member of the channel by tagging them (@member's name). As you can see on the screenshot attached, when a message with a tag shows the email icon it means the tagged user has received an email. The option to create a channel for all-email communications however is no longer available.
As for private channels, you can either create a private group chat or create a dedicated group under Settings > Users and Companies > Groups, and add it to the channel's privacy settings (see screenshot attached).
Other Odoo users have contacted us about the mailing list feature and feedback requests have been created for the option to be available again in future versions, so hopefully we will see this option back soon.
I hope you find this information useful, let me know if you have any more questions.