With macOS Ventura, you can mark conversations as unread so you can return to them later when you have time to respond.
How to mark conversations as unread
In the Messages app, select a conversation
Do any of the following:
In the menu bar, choose Conversation > Mark as Unread
Control-click the conversation, then choose Mark as Unread. Or, if you have a trackpad or a Magic Mouse, you can swipe right with two fingers on the conversation in the sidebar, then click the Mark as Unread button