How can I find deleted Apple Mail attachments in Time Machine?

Think of a date before you deleted them and, using Finder, go to the corresponding backup.

In Big Sur, each backup looks like a separate disk inside you TM disk. In Catalina it is a folder in your TM disk.

Before going further, make sure that Finder is showing you hidden files and folders. Command-Shift-Dot to enable this.

Within the chosen dated folder drill down to your Mail folder which is inside your username's Library. Like this:

If you have more than one mail account (like me) there will be multiple folders within V8. You will have to try each one.

Going deeper:

First go down folders just like the folders within Mail, but below the folder Data, it is just guess work.

Finally there are folders for a few Messages and their Attachments.

Explore!

When you do find the lost attachments, just copy (drag and drop) them to your Desktop or wherever is convenient.

In case you are wondering, I have decided there is nothing sensitive in the names of my folders and so have not attempted to hide anything in my examples.