That solely depends on your use case and if its something that you wish to allow. In secure environments (think Banking, Hospitals, and other regulated industries) you would absolutely block ms-rdp connects to your untrust interface. The risk in allowing RDP connections to external resources comes more in data retention and access rather than it being inheritably insecure. It's an easy way at that point for data to be exfiled from your network either for malicious purposes or by a well-meaning technically savvy user. In these type of environments though, it's absolutely something you wouldn't want to allow.
In a more office centric environment where you don't have any regulatory compliance reasons to limit this access and didn't have a strong business need to secure data, then this is more of a policy decision. I would argue that your employees probably have no need to ever access any outside resources via RDP, and that it can safely be closed and requests to open access can be evaluated for business need on a case-by-case basis.
Essentially, unless you have a business need for this to be open to the outside work you provide a chance for data exfiltration that likely doesn't have a valid reason to be allowed.