Remove old modern mode experiment

[Timer]

This PR removes the modern mode experiment because:

• It does not yield meaningful bundle size wins when compared to other initiatives we've taken
• It's not compatible with webpack 5 (which we're upgrading to)
• It's currently broken and causes most apps to malfunction
• There's no champion currently owning the experiment

We can re-introduce this in the future when we'd like to make it a default for all Next.js apps.

Note: Next.js still supports Differential Loading (nomodule) and does it by default. This PR strictly removes the experimental modern syntax, and does not disable our existing modern/legacy polyfilling.

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Fixes #19200
Fixes #18960
Fixes #14707
Fixes #14465

[ijjk]

Stats from current PR

Default Server Mode (Decrease detected ✓)

General Overall decrease ✓

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
buildDuration	10.2s	10.1s	-92ms
nodeModulesSize	85 MB	84.9 MB	-52.8 kB

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.216	2.231	⚠️ +0.01
/ avg req/sec	1128.39	1120.6	⚠️ -7.79
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.229	1.304	⚠️ +0.07
/error-in-render avg req/sec	2034.46	1916.68	⚠️ -117.78

Client Bundles (main, webpack, commons)

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
677f882d2ed8..4972.js gzip	12.7 kB	12.7 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-f1a49fb..e45e.js gzip	6.52 kB	6.52 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.9 kB	58.9 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
_app-3b0cf13..85f8.js gzip	1.28 kB	1.28 kB	✓
_error-6f635..c393.js gzip	3.44 kB	3.44 kB	✓
hooks-d4ffc3..9e0f.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-b618194..5477.js gzip	1.61 kB	1.61 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	8.01 kB	8.01 kB	✓

Client Build Manifests

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
_buildManifest.js gzip	321 B	321 B	✓
Overall change	321 B	321 B	✓

Rendered Page Sizes

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
index.html gzip	613 B	613 B	✓
link.html gzip	621 B	621 B	✓
withRouter.html gzip	608 B	608 B	✓
Overall change	1.84 kB	1.84 kB	✓

---

Serverless Mode (Decrease detected ✓)

General Overall decrease ✓

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
buildDuration	12.1s	11.7s	-372ms
nodeModulesSize	85 MB	84.9 MB	-52.8 kB

Client Bundles (main, webpack, commons)

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
677f882d2ed8..4972.js gzip	12.7 kB	12.7 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-f1a49fb..e45e.js gzip	6.52 kB	6.52 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.9 kB	58.9 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
_app-3b0cf13..85f8.js gzip	1.28 kB	1.28 kB	✓
_error-6f635..c393.js gzip	3.44 kB	3.44 kB	✓
hooks-d4ffc3..9e0f.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-b618194..5477.js gzip	1.61 kB	1.61 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	8.01 kB	8.01 kB	✓

Client Build Manifests

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
_buildManifest.js gzip	321 B	321 B	✓
Overall change	321 B	321 B	✓

Serverless bundles Overall decrease ✓

	vercel/next.js canary	Timer/next.js remove/modern-mode	Change
_error.js	915 kB	914 kB	-687 B
404.html	2.67 kB	2.67 kB	✓
hooks.html	1.92 kB	1.92 kB	✓
index.js	915 kB	915 kB	-687 B
link.js	974 kB	973 kB	-687 B
routerDirect.js	966 kB	966 kB	-687 B
withRouter.js	966 kB	966 kB	-687 B
Overall change	4.74 MB	4.74 MB	-3.44 kB

Commit: 91289dd

[franklinjavier]

Awesome

[PepijnSenders]

It does not yield meaningful bundle size wins when compared to other initiatives we've taken

When we implemented this we saw a 3%-6% improvement in bundle size, which other initiatives can we use to achieve the same @Timer ?

[developit]

Would be nice to see a replacement for this given the effort involved in removing it (rather than updating it). Modern mode shaves 21% off the initial page weight for Next.js apps using Preact, independently of the savings attributable to the nomodule polyfill chunk.

[DoctorDerek]

Would be nice to see a replacement for this given the effort involved in removing it (rather than updating it). Modern mode shaves 21% off the initial page weight for Next.js apps using Preact, independently of the savings attributable to the nomodule polyfill chunk.

Yeah I love this concept and think it should be a core feature, but it seems like the incompatibility with the upgrade to Webpack 5 is a dealbreaker. Any thoughts on how we would reimplement this feature in a compatible way @developit?