Any songs you have added through apple Music since joining AM (minus something you may have bought through iTunes) will go away. They may not actually go away, just may become unplayable. Based on how match worked, you would still be able to get access to those songs if you sign back up to AM within a certain amount of time.
There is also a chance that you may lose all songs in your library so yes best to have a copy of anything that you had prior to joining AM