@paj28 and I have been corresponding about this over the mailing list. Creating a ticket here so that it gets fixed at some point.

I'm trying to understand what has been done around "required" and
RelatedItemValidator and RelatedOneToOneValidator. For the rest of
Toscawidgets, widgets don't have a required parameter, instead you set
"validator = twc.Required" - or if you want to further customise the
validator "validator = twc.MyValidator(required=True)". I would prefer to
keep things consistent.

This is very recent and an oversight on my part for not seeing the
break in design symmetry. I say go right ahead with the refactor at
your leisure.

I had a go at this, and I'm struggling to do it and keep the tests passing!
I will try some more when I get some time, or if you fancy having a go,
that'd be fab.

I think this was introduced in Pull Request #6.