Turbopack builds do not correctly process all files (ex: turbopack runtime) through the browserslist system

Link to the code that reproduces this issue

https://github.com/krzkz94/turbopack-modern-syntax-browserslist

To Reproduce

Clone the repository
https://github.com/krzkz94/turbopack-modern-syntax-browserslist

Install dependencies
npm i

Run the build
npm run build

Run the following command in the build folder which will grep all modern syntax
chmod +x ./check-failures-only.sh && ./check-failures-only.sh

The above is a simple helper tool to identify transpilation issues, only runs 'grep' against the most common modern syntax I could think of.

According to the default browserslist defined here
https://nextjs.org/docs/architecture/supported-browsers

The syntax below is not being transformed
Optional Chaining (?.)
Introduced in: Chrome 80, Edge 80, Firefox 72, Opera 67, Safari 13.1
Nullish Coalescing (??)
Introduced in: Chrome 80, Edge 80, Firefox 72, Opera 67, Safari 13.1
Nullish Assignment (??=)
Introduced in: Chrome 85, Edge 85, Firefox 79, Opera 71, Safari 14
Private Class Fields (#field)
Introduced in: Chrome 74, Edge 79, Firefox 90, Opera 62, Safari 14.1
globalThis
Introduced in: Chrome 71, Edge 79, Firefox 65, Opera 58, Safari 12.1

One thing to note, the turbopack runtime is not transformed either (when it should)

Can a member of the Next/Turbopack team confirm or deny that every dependency (also transient dependencies) should be ran through the browserslist, even if it's not part of transpilePackages?
The majority of nextjs compatibility issues comes from sub-dependencies (even 4 levels deep), so if you want to make your site compatible with for example smart tv's from 2022 (which run chrome 68), it's basically impossible as your transpilePackages would have to be thousands of lines long.
I know for webpack at least, transpilePackages ignores sub-dependencies.

Current vs. Expected behavior

Currently, a lot of modern syntax is not ran through the browserslist compatibility / polyfill / transpilation logic

Expected: All modern syntax that doesn't match the browserslist should be transformed.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.0.0: Mon Aug 25 21:17:54 PDT 2025; root:xnu-12377.1.9~3/RELEASE_ARM64_T6041
  Available memory (MB): 65536
  Available CPU cores: 16
Binaries:
  Node: 22.12.0
  npm: 10.9.0
  Yarn: N/A
  pnpm: 8.15.9
Relevant Packages:
  next: 15.5.3 // Latest available version is detected (15.5.3).
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.9.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Turbopack, Runtime, Webpack

Which stage(s) are affected? (Select all that apply)

next build (local), next dev (local)

Additional context

No response

_PACK-5450

Clone the linked reproduction repository, install dependencies with npm i, and run npm run build. Use check-failures-only.sh to inspect untransformed syntax and compare the generated output with the stated browserslist; done means the affected build and runtime files are transformed consistently for the target browsers.