user_attach_accounts() and comment_prepare_author() have been removed

Both functions used entity_create() to create temporary, anonymous user entities while viewing nodes and comments. That was very slow and the corresponding code has been refactored to improve performance.

Instead of calling user_attach_accounts(), the author formatter that is used for displaying node authors now extends from EntityReferenceFormatterBase, which has the functionality to preload user entities built in. This is the recommended approach, if that is not an option, then it is possible to manually load the user entities like CommentViewBuilder still does.

comment_prepare_author() has been merged into $comment->getOwner().

Before

$author = comment_prepare_author($comment->getOwner());

After

$author = $comment->getOwner();