This plugin does exactly what it needs to do and is very simple. Basically, any custom posts of custom post types no longer active are listed under Settings > Remove Oldies, and then there is a button that just removes them from the DB.
Here are just some suggestions that maybe could be considered for the plugin:
1) Maybe do a little more with the interface to pretty it up (I think you mentioned you were already doing this). -- But you could maybe have a standard table on this page where it displays the id of the post type, the number of posts it has in the DB, and the "remove" button.
2) I know the name of the plugin is "Remove Oldies" but maybe it could still have the active post types listed in that table, as well, for a better overview. Maybe there could be some kind of indication on the table to which ones are active or not. Or maybe the "Remove" button could be hidden for the active post types with a message that just says "Active". -- And so they can see they'd have to make that post type not active in order to remove those.
3) I think I saw you were working on a separate one for deleting custom fields. Maybe consider just making this part of the process of removing the posts. I'm not sure why you'd ever delete a post but leave its associated custom field data.
Anyway, thanks for putting the plugin together! It's a very cool plugin in a time where we've all run rampid with custom post type fever in our themes and plugins.