GitHub checks for a bunch of features to determine if a browser is supported.

You can see these checks yourself at https://github.com/github/browser-support/blob/main/src/index.ts.
However this might not exactly match the deployed version, which you can see in browser dev-tools through source maps at assets/node_modules/@github/browser-support/lib/index.js

Base support:

• Blob: #14563
• PerformanceObserver: #20760
• Intl: #9633
• MutationObserver: #14559
• URLSearchParams
• WebSocket
• IntersectionObserver: #19860
• queueMicrotask
• TextEncoder
• TextDecoder
• customElements
• <details>: #20965
• AbortController
• AbortSignal
• globalThis
• FormData.prototype.entries
• Element.prototype.toggleAttribute: #13360
• Object.fromEntries
• Array.prototype.flatMap
• String.prototype.trimEnd
• Promise.allSettled
• String.prototype.matchAll
• String.prototype.replaceAll
• Promise.any
• AggregateError
• Element.prototype.replaceChildren

Polyfilled:

• AbortSignal.abort: #23279
• AbortSignal.timeout: #23354
• Array.prototype.at
• String.prototype.at
• TypedArray.prototype.at
• crypto.randomUuid: #13356
• Element.prototype.checkVisibility
• signal option of EventTarget.prototype.addEventListener
• HTMLFormElement.prototype.requestSubmit: #23204
• Object.hasOwn
• Cooperative Scheduling of Background Tasks: #13413
  • requestIdleCallback
  • cancelIdleCallback
  • IdleDeadline
• Array.prototype.findLast
• Array.prototype.findLastIndex

Required features without feature tests:

• Element.prototype.attachShadow
• Generator functions
• fetch: #15795