IIRC the same thing happened with 2018 / 2015? Could this be autodesk enforcing the 'we only support the previous 2 releases' mantra? Although i think there were workarounds through reinstalling sequence. I had a 2015 project that was hamstrung by it.
Kinda blows either way.

Whenever John and I talk about graphs, this comes up. Mainly because I do my template work in 2016 and then make updated copies.

But, when John and I talk about it, it's usually in the context of: there are a ton of limitations on the 2016 API that don't exist in the later versions, so a lot of things that work in 17/18/19 just don't work in 16.

I'm not saying that's why Autodesk isn't making new Dynamo support 16, but I can safely say that the majority of useful graphs we've made recently, won't work in 16 anyway.

Having said that... I PERSONALLY think you can install them side by side. I am pretty sure they will run. But I'm not sure they won't have underlying issues, while running. (python or depenancy issues...)

Sent from my Phablet. Please excuse typos... and bad ideas.

Aaron Maller
Director
Parallax Team, Inc.