Is there a way, by means of some setting or programmatically, to allow a user on Slack to confirm that the user really wants to send a message before the message is sent? Often times we end up sending messages on the wrong channels or DMs. How can we prevent these?
Assuming by "send a message" you mean "post a message" to a channel.
No. There is no Slack setting to enable confirmation before a message is posting.
No. Its possible to retrieve all messages that are posted to a channel with a Slack app (e.g. via Events API) and then react to them, but only after the fact. To the best of my knowledge it is not possible to insert some kind of confirmation logic into the standard process of posting a message.