Goodbyes are optional and understated by design: a small note, not a broadcast.
Pick a goodbye channel and write the farewell line on the same settings page as welcomes. Placeholders and image cards work here too. The default is a simple note that the member has left.

If an unusual burst of members joins at once, the bot pauses welcomes, posts a raid warning and pings your moderator role, so a raid never gets a hundred warm greetings. This works alongside the dedicated Anti-Raid module.