[Fiber] Delete isMounted internals

[sebmarkbage]

The public API has been deleted a long time ago so this should be unused unless it's used by hacks. It should be replaced with an effect/lifecycle that manually tracks this if you need it.

The problem with this API is how the timing implemented because it requires Placement/Hydration flags to be cleared too early. In fact, that's why we also have a separate PlacementDEV flag that works differently.

https://github.com/facebook/react/blob/main/packages/react-reconciler/src/ReactFiberCommitWork.js#L2157-L2165

We should be able to remove this code now.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
react-compiler-playground	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jan 3, 2025 3:28am

[react-sizebot]

Comparing: 3b009b4...e650612

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	=	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	512.42 kB	512.26 kB	=	91.59 kB	91.57 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	517.20 kB	517.04 kB	=	92.44 kB	92.42 kB
facebook-www/ReactDOM-prod.classic.js	=	594.22 kB	593.89 kB	=	104.67 kB	104.62 kB
facebook-www/ReactDOM-prod.modern.js	=	584.49 kB	584.32 kB	=	103.12 kB	103.09 kB
oss-experimental/react-reconciler/cjs/react-reconciler-reflection.production.js	=	5.69 kB	5.44 kB	=	1.53 kB	1.48 kB
oss-stable-semver/react-reconciler/cjs/react-reconciler-reflection.production.js	=	5.69 kB	5.44 kB	=	1.53 kB	1.48 kB
oss-stable/react-reconciler/cjs/react-reconciler-reflection.production.js	=	5.69 kB	5.44 kB	=	1.53 kB	1.48 kB
oss-experimental/react-reconciler/cjs/react-reconciler-reflection.development.js	=	6.80 kB	6.00 kB	=	1.66 kB	1.48 kB
oss-stable-semver/react-reconciler/cjs/react-reconciler-reflection.development.js	=	6.80 kB	6.00 kB	=	1.66 kB	1.48 kB
oss-stable/react-reconciler/cjs/react-reconciler-reflection.development.js	=	6.80 kB	6.00 kB	=	1.66 kB	1.48 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-experimental/react-art/cjs/react-art.development.js	=	594.02 kB	592.80 kB	=	94.86 kB	94.58 kB
facebook-www/ReactTestRenderer-dev.classic.js	=	572.97 kB	571.79 kB	=	93.50 kB	93.21 kB
facebook-www/ReactTestRenderer-dev.modern.js	=	572.97 kB	571.79 kB	=	93.50 kB	93.21 kB
oss-stable/react-art/cjs/react-art.development.js	=	554.32 kB	553.10 kB	=	89.36 kB	89.01 kB
oss-stable-semver/react-art/cjs/react-art.development.js	=	554.24 kB	553.02 kB	=	89.34 kB	88.99 kB
facebook-www/ReactART-prod.modern.js	=	357.10 kB	356.27 kB	=	59.80 kB	59.67 kB
facebook-www/ReactART-dev.classic.js	=	648.92 kB	647.32 kB	=	103.22 kB	102.73 kB
oss-experimental/react-art/cjs/react-art.production.js	=	305.56 kB	304.73 kB	=	52.20 kB	52.07 kB
oss-stable/react-art/cjs/react-art.production.js	=	301.19 kB	300.36 kB	=	51.31 kB	51.18 kB
oss-stable-semver/react-art/cjs/react-art.production.js	=	301.11 kB	300.28 kB	=	51.29 kB	51.16 kB
test_utils/ReactAllWarnings.js	=	63.02 kB	62.74 kB	=	15.68 kB	15.67 kB
oss-experimental/react-reconciler/cjs/react-reconciler-reflection.production.js	=	5.69 kB	5.44 kB	=	1.53 kB	1.48 kB
oss-stable-semver/react-reconciler/cjs/react-reconciler-reflection.production.js	=	5.69 kB	5.44 kB	=	1.53 kB	1.48 kB
oss-stable/react-reconciler/cjs/react-reconciler-reflection.production.js	=	5.69 kB	5.44 kB	=	1.53 kB	1.48 kB
oss-experimental/react-reconciler/cjs/react-reconciler-reflection.development.js	=	6.80 kB	6.00 kB	=	1.66 kB	1.48 kB
oss-stable-semver/react-reconciler/cjs/react-reconciler-reflection.development.js	=	6.80 kB	6.00 kB	=	1.66 kB	1.48 kB
oss-stable/react-reconciler/cjs/react-reconciler-reflection.development.js	=	6.80 kB	6.00 kB	=	1.66 kB	1.48 kB

Generated by 🚫 dangerJS against 435d3c9

[eps1lon]

We should remove it here as well or update the message to say it has been removed:

react/packages/react/src/ReactBaseClasses.js

Lines 96 to 100 in d94334a

	isMounted: [
	'isMounted',
	'Instead, make sure to clean up subscriptions and pending requests in ' +
	'componentWillUnmount to prevent memory leaks.',
	],