How to get stationery back in Apple Mail on macOS Mojave?
You must have a copy of the following files from a computer running High Sierra (10.13) or lower In this example I used files from Sierra (10.12)


/Applications/Mail/Contents/Resources/ComposeWindow.toolbar
/Library/Application Support/Apple/Mail


If you would like the stationery in the original mail application then skip step 1, disable SIP, and continue with the original application, SIP can be reenabled after. Make copies of everything. Instead of "replace" add a .orig to the file extension.


Duplicate the Mail Application
Right click on the new Mail Application and go to /Contents/Resources/
Replace the ComposeWindow.toolbar with the ComposeWindow.toolbar from the copied files
Go to Finder>Go To Folder>/Library/Application Support/Apple/
Copy the Mail folder from the copied files to this folder
Open the Mail application and compose a new email
Right Click on the Toolbar and select Customize Toolbar
Drag the icon labeled Show Stationery to your toolbar and hit done
Click the Show Stationery button in your toolbar 


Note:  The toolbar item DOES work with dark mode but the Stationery Panel does not.



macOS Sierra (10.12) Files are available on Github → https://github.com/jbis9051/StationeryMailFiles