Since both of these are heading into a kind of Legacy mode, is there anyway these can be Open Sourced, and up on Github, so that those of us that are going to be reliant on it for years to come can at least look into it and optimise and/or extend if/where/what we can?
Both UGUI and TextMesh Pro source code is already available to you under Unity Companion License Unity Companion License ("License") .
It’s probably not considered Open Source by the definitions of OSI or Free Software Foundation. But it does allow you to read the source and even create derivative works (with some restrictions see the license text for exact details).
You can find the source code in the Library folder of your project or by simply using Go to definition functionality in your IDE. There is a github repository but it hasn’t been updated for a few years, so even though UGUI hasn’t changed too much in that time better use the version of code from Library folder for latest bugfixes and best compatibility with your specific Unity version.
I don’t know if there are any hidden problems that would show up if you tried to create and use full copy of those libraries due to Unity using some internal APIs you can’t, or the Editor itself having some hardcoded interactions expecting the original UGUI version. But for the most part is straight forward C# code. And just being able to read it gives you quite a bit control by manipulating the inputs or subclassing some of the components. Even if you can’t create a modified version of whole library, copy pasting and renaming individual classes to avoid conflicts should work for large portions of it.
Is it sufficiently accessible that it’d be possible to (in place) Burst-ify and SIMD considerately optimise the constraints solvers of UGUI/Canvases etc?