Add deprecation notices to the Upgrade guide

docs/source/upgrade-guide/index.md

@@ -522,6 +522,144 @@ The `react/jsx-key` rule has been enabled in ESlint for catching missing `key` i
 You might catch some violations in your project or add-on code after running ESlint.
 Adding the missing `key` property whenever the violation is reported will fix it.
 
+### Deprecation notices for Volto 18
+
+#### `@plone/generator-volto`
+
+```{deprecated} Volto 18.0.0
+```
+
+The Node.js-based Volto project boilerplate generator is deprecated from Volto 18 onwards.
+After the release of Volto 18, it will be marked as deprecated, archived, and it won't receive any further updates.
+Although you can still migrate your project to Volto 18 using this boilerplate, you should migrate to using [Cookieplone](https://github.com/plone/cookieplone).
+
+##### Alternative
+
+Migrate your project to use a [Cookieplone](https://github.com/plone/cookieplone) boilerplate.
+
+#### Volto project configurations
+
+```{deprecated} Volto 18.0.0
+```
+
+Configuring Volto using {file}`src/config.js` at the project level is deprecated in Volto 18, and will be removed in Volto 19.
+
+```{seealso}
+See https://github.com/plone/volto/issues/6396 for details.
+```
+
+##### Alternative
+
+You should configure your projects in a policy add-on.
+You can move your project to use [Cookieplone](https://github.com/plone/cookieplone) which provides the necessary boilerplate for it.
+
+#### Semantic UI
+
+```{deprecated} Volto 18.0.0
+```
+
+The Semantic UI library is not maintained anymore, and will be removed in Plone 7.
+You should no longer use Semantic UI in add-ons and projects.
+
+```{seealso}
+Related PLIPs:
+
+- https://github.com/plone/volto/issues/6321
+- https://github.com/plone/volto/issues/6323
+
+```
+
+##### Alternatives
+
+You can use any supported component framework of your choice for implementing new components, especially in the public theme side.
+If you create new widgets or components for the CMSUI—in other words, the non-public side—you should use the [`@plone/components`](https://github.com/plone/volto/tree/main/packages/components) library as an alternative.
+Even though it's still in the development phase, it will be completed in the next few months, and will be supported in the future.
+
+#### `lodash` library
+
+```{deprecated} Volto 18.0.0
+```
+
+`lodash` is deprecated in Volto 18, and will be removed in Plone 7.
+
+`lodash` has not received any updates since 2021.
+It has performance issues from bloated bundles and it's not prepared for ESM.
+Lots of `lodash` utility helpers can be replaced with vanilla ES.
+These issues cause concern about its future maintainability.
+
+In place of `lodash`, Plone 7 will use both the `lodash-es` library, which is ESM ready, and modern vanilla ES alternatives whenever possible.
+
+##### Alternatives
+
+```{seealso}
+The following links suggest alternatives to `lodash`.
+
+-   https://github.com/you-dont-need/You-Dont-Need-Lodash-Underscore
+-   https://javascript.plainenglish.io/you-dont-need-lodash-how-i-gave-up-lodash-693c8b96a07c
+
+If you still need some of the utilities in `lodash` and cannot use vanilla ES, you can use `lodash-es` instead.
+```
+
+#### `@loadable/component` and Volto `Loadables` framework
+
+```{deprecated} Volto 18.0.0
+```
+
+`@loadable/component` and the Volto `Loadables` framework is deprecated in Volto 18, and will be removed from Plone 7.
+It's a Webpack-only library, and it does not have Vite plugin support.
+Since React 18, this library is no longer necessary, as it has the initial implementation of the "concurrent mode".
+React 19 will further improve it and add more features around it.
+
+##### Alternatives
+
+Use plain React 18 lazy load features and its idioms for lazy load components.
+
+```jsx
+const myLazyComponent = lazy(()=> import('@plone/volto/components/theme/MyLazyComponent/MyLazyComponent'))
+
+const RandomComponent = (props) => (
+    <React.Suspense>
+        <MyLazyComponent />
+    </React.Suspense>
+)
+```
+
+There's no support for pre-loading or lazy loading entire libraries as in `@loadable/component`.
+With the removal of barrel imports files, as described in the next deprecation notice, it is now unnecessary.
+
+#### Removal of barrel import files
+
+```{deprecated} Volto 18.0.0
+```
+
+Volto previously used barrel imports, which are centralized files where other imports are re-exported, to improve the developer user experience.
+With barrel imports, a developer only needs to remember to import from the re-exported place, not the full path.
+
+It became a bad practice, because bundlers rely upon the import path to determine whether to bundle code together or not.
+Since the barrel imports direct import all the code, a lot of imports ended up in the same main chunk of code.
+Modern bundlers like Vite also use that technique, so it heavily relies on this.
+The barrel imports must be removed to increase the natural number of chunks that Volto divides on—especially on routes—resulting in code splitting done the right and natural way.
+
+This forces us to rewrite all the imports everywhere—including core, projects, and add-ons—once we implement it.
+
+The barrel imports files include the following.
+
+-   {file}`src/components/index.js`
+-   {file}`src/helpers/index.js`
+-   {file}`src/actions/index.js`
+
+##### Alternative
+
+Implement only direct imports in code, preparing now for the upcoming change.
+
+```diff
+-import { BodyClass } from '@plone/volto/helpers';
++import BodyClass from '@plone/volto/helpers/BodyClass/BodyClass';
+```
+
+Once this is implemented, a code modification will be provided for a smooth migration.
+
+
 (volto-upgrade-guide-17.x.x)=
 
 ## Upgrading to Volto 17.x.x