You are not the first to be confused: Confused on cardinality for Protocol - #3 by sebastian.garde
In ADL 1.4, an unstated existence historically defaulted to 1..1 (mandatory) as you noted. However, modern interpretations and ADL 2 derive effective constraints and defaults directly from the underlying Reference Model (RM) or treat unstated constraints as optional/implicit.
If you want your ADL parser to be ready for ADL2, I suggest you also use the RM during parsing ADL 1.4. The defaults for HISTORY.events should be taken from the RM HISTORY class where they are optional.
Check the ADL 2.4 (experimental) radio button on the bottom of ADL tab in CKM. There is also ANTLR4 for ADL2.