Yes, the feature still exists and actually is part of all the templates now.
enable_category_multi_select=true to the shortcode to enable it.
If you want a submit button, see this:
The code will have to be very slightly tweaked depending on which template you are using. Which template are you using?
As for changing it to ‘OR’, sorry, no, not possible because the code that create the query does not support it at this time.
Hope that helps!