As mentioned in that thread the status was primarily “wont fix”, however the concession was to add a basic feature to the pre-release version (eventually available with stable version 3.0 onwards), which AFAIK should still work the same way.
Quoted from https://code.google.com/archive/p/sumatrapdf/issues/2621:
Thanks for the suggestion. I’ve created an experimental implementation of something along your description with r9022.
Please download a pre-release build from https://www.sumatrapdfreader.org/prerelease.html (revision 9022 or later once it’s available), open a document and then hit the M key (for “Measurement”) to see what we’ve currently got - and keep hitting M for changing the measurement unit. To get even closer to an actual ruler, try selecting part of the page while keeping the Ctrl key pressed.
It is very rudimentary and I along with others wanted more, but I have often used it as it’s better than nothing. Perhaps other helper apps mentioned in that same thread are more useful, but I have not tested them recently.