The PR form has a bunch of quality-of-life features that would be impossible without JavaScript. The reviewers/assignees/labels pickers are JS-based and fetch data on the fly, the Markdown editor with previews also couldn’t exist with JS. Many of those components appear in other pages, and it’s a no-brainer that GitHub wants to reuse them between pages. GitHub picked a solution for this that is supported (at least partially) by browsers with a total 94% market share [1].
Would it be possible to make a form with all those features and that works (at least with 95% of the features) with your legacy (dead) browser of choice, be it IE6 or Pale Moon? Sure, it would. But to do that, GitHub would need to spend a lot of resources, even though most GitHub users do not need those dead browsers. Those browsers don’t support many modern APIs that web apps need, or that make developers’ lives easier.
Speaking of dead software, Pale Moon dropped support for Windows XP in 2016 [2]. What happened? Why does software drop support for old runtime environments? Because nobody has the time and resources to test things on XP, and to write polyfills for features that XP does not support, or to skip some features because they don’t want XP users to get a worse browser, and because new features can make your software better for the user or more secure.
The world has moved on. Install a modern version of Firefox (or Chrome/ium if you must) and stop complaining.
Would it be possible to make a form with all those features and that works (at least with 95% of the features) with your legacy (dead) browser of choice, be it IE6 or Pale Moon? Sure, it would. But to do that, GitHub would need to spend a lot of resources, even though most GitHub users do not need those dead browsers. Those browsers don’t support many modern APIs that web apps need, or that make developers’ lives easier.
Speaking of dead software, Pale Moon dropped support for Windows XP in 2016 [2]. What happened? Why does software drop support for old runtime environments? Because nobody has the time and resources to test things on XP, and to write polyfills for features that XP does not support, or to skip some features because they don’t want XP users to get a worse browser, and because new features can make your software better for the user or more secure.
The world has moved on. Install a modern version of Firefox (or Chrome/ium if you must) and stop complaining.
[1]: https://caniuse.com/?search=components
[2]: https://en.wikipedia.org/wiki/Pale_Moon_(web_browser)#Releas...