Getting worse.....
All autosaves are numbered across all used characters.
ie. Tav1 gets Autosave_0 , 1, 2.
Tav2 gets 3, 4 ,5

Guess what happens when maximum autosaves is reached....
Yup, Tav1 will be overwritten.

So Larian can name " You can now delete save files by playthrough." as a highlight, but you really don't have to bother. The saves will be deleted for you.