What are WordPress admin supported browsers?

The "Official" supported browsers list is currently here:

• https://make.wordpress.org/design/handbook/design-guide/browser-support/

You can also use https://www.npmjs.com/package/browserslist-config-wordpress if you use PostCSS, Autoprefixer, stylelint etc

Supported browsers will change with the each release and theme you have installed. You might want to check the release notes or issues on trac to find any incompatibilities.

The best I could find was Browse Happy but nothing in the commits narrowed down or showed version support info.

So you might be able to tie latest browser support with how far back you want to support - https://www.impressivewebs.com/release-history-major-browsers/

Based on this answer, you might be able to tie this all back to If your menus or widgets screens broke…

WordPress 3.0.5 was released at the same time as jQuery 1.5. Unfortunately, 1.5 has some backwards incompatible changes that appear to break a number of areas in the admin. The timing is awkward and it looks like it was us. It wasn’t.

There’s nothing we can do about this even for WordPress 3.1, which is freezing at jQuery 1.4.4.

If your theme deregisters jQuery and re-registers jQuery 1.5, then you’ll want to make sure that this change only applies for the frontend, i.e. ! is_admin(). (Or use the wp_enqueue_scripts hook, which only fires for the theme-side.) This might not be obvious — if you’re enqueueing the latest jQuery from, say, Google’s CDN, you’ll be getting 1.5 suddenly, and things will break.

Someone should put together a quick plugin that restores and enforces the bundled jQuery in the admin. I’ll do it later tonight if no one else does.

Reference: #16508 and numerous support forum threads.

But 6 months ago, jQuery.js was updated to 1.12.4 which shows support for Safari 9 and IE 11 in the changelog.