I am interested in running Ubuntu 16.04 on Windows Subsystem for Linux, as it is required for a certain build environment. All that I can find on Microsoft Store is 18.04 and 20.04. Is there a way I can install from iso potentially?
Refer to this article for instructions regarding how to manually download Windows Subsystem for Linux distro packages that aren't in the Microsoft store (this includes Ubuntu 16.04).
As @user535733 pointed out in the comments, WSL is a proprietary service created by Microsoft. Therefore, you will be able to do more by either using a virtual machine (VMWare or VirtualBox) or by dual booting Ubuntu with your existing windows installation.