According to help:
To follow the syntax with all the other commands, I'd assume it should be:
Checks whether a variable's contents match one of the items in a list.
IfInList, Var, MatchList
IfNotInList, Var, MatchList
IfContainsList, Var, MatchList
IfNotContainsList, Var, MatchList
That way it is clear to the user it cannot be combined with booleans. Further, this change could be entirely backwards compatible, all previous usages in scripts would work, but so would the above, and the above would be what is in the documentation.
Now, I know there's this line in the documentation:
The operators "between", "is", "in", and "contains" are not supported in expressions.
But the wording is confusing. If for whatever reason you don't want to update the syntax, you should at least give an example in the help to make it more clear that booleans won't work, such as