In fact, it is the opposite - any to-one relation field can be made required, but to-many relation fields cannot.
Not sure which side of the relation filter you are referring to, but in general a dynamic relation filter can take either of these forms:
To-one field ... is ... This entity → Field value
or
Collection field ... contains ... This entity → Field value
But the right hand side field value needs to be a single entity (a to-one relation).
You can vote here for the option to support
To-one field ... is any of ... This entity → Collection field
At the risk of being pedantic, there is no such thing as a ‘to-many relationship’. The whiteboard can show relations which are m:m, 1:m or m:1 but it is indeed not currently possible to show 1:1 relations.