Does "apt autoremove --purge <package>" also remove configuration files for the related packages that are removed?
Short Answer : Yes, but not every file related to package is deleted as purge doesn't remove data stored by packages inside the home directory or other special directories.
By Standard definition of Purge:-
Purge - Purge is identical to remove except that packages are removed and purged. Purge meaning that any configuration files are deleted too.