Softball/Rule on sliding
Is there an actual written rule that requires a runner to slide into a base for any play; only close plays; or no specific written rule but rather left up to interpretation/judgement of the umpire?
There is no rule requiring a player to slide. It is not left up to the judgement of the umpire. An umpire can not put a player out for not sliding.
If a runner plows into a player in possession of the ball they could be called out. If is the runners responsibility to avoid excessive contact of a player in possession of the ball. Sliding is the best way to avoid this contact.