SO MANY Upwork freelancers have great portfolios on their Upwork profile page. That's all I need when I hire artists and designers.
I don't have any need to consider the freelancers who don't bother to post a portfolio. I just assume they're not serious about working here.
Thanks for asking this question Bryan. As a designer I thought employers can look at the work I did to other freelancers which is not good for privacy. Those answers were the best.