Adding notes on modern support on classic extensibility options

Author: VesaJuvonen

docs/solution-guidance/Customize-a-SharePoint-page-by-using-remote-provisioning-and-CSS.md

@@ -1,7 +1,7 @@
 ---
 title: Customize a SharePoint page by using remote provisioning and CSS
 description: Use CSS to customize SharePoint rich text fields and web part zones.
-ms.date: 4/25/2018
+ms.date: 4/29/2019
 localization_priority: Priority
 ---
 
@@ -11,6 +11,9 @@ You can use cascading style sheets (CSS) to customize SharePoint rich text field
 
 For a code sample that is associated with this article, see [Branding.AlternateCSSAndSiteLogo](https://github.com/SharePoint/PnP/tree/master/Samples/Branding.AlternateCSSAndSiteLogo) in the Office 365 Developer Patterns and Practices project on GitHub.
 
+> [!IMPORTANT] 
+> This extensibility option is **only** available for classic SharePoint experiences. You cannot use this option with modern experiences in SharePoint Online, like with communication sites. Notice also that you should **not** take a dependency on the html page structure or on the out-of-the-box CSS style names as these might be adjusted without a notice.
+
 ## Customize rich text fields
 
 You can customize rich text fields by using CSS right in the page editor:

docs/solution-guidance/Customize-your-SharePoint-site-UI-by-using-JavaScript.md

@@ -1,13 +1,16 @@
 ---
 title: Customize your SharePoint site UI by using JavaScript
-ms.date: 11/03/2017
+ms.date: 04/29/2019
 localization_priority: Priority
 ---
 # Customize your SharePoint site UI by using JavaScript
 
 You can update your SharePoint site's UI by using JavaScript.
 
 _**Applies to:** Office 365 | SharePoint 2013 | SharePoint Online_
+
+> [!IMPORTANT] 
+> This extensibility option is **only** available for classic SharePoint experiences. You cannot use this option with modern experiences in SharePoint Online, like with communication sites. Notice also that you should **not** take a dependency on the html page structure or on the out-of-the-box CSS style names as these might be adjusted without a notice.
     
 The [Core.EmbedJavaScript](https://github.com/SharePoint/PnP/tree/master/Samples/Core.EmbedJavaScript) sample add-in adds a status bar message to all pages on a SharePoint site, and removes the **new subsite** link from the **Site Contents** page by using JavaScript. 
 

docs/solution-guidance/branding-sharepoint-sites-sharepoint-add-in.md

@@ -13,6 +13,9 @@ The approach you take to brand SharePoint sites is different in the new SharePoi
 
 In a SharePoint Add-in model branding scenario, all of the options associated with FTC / Farm Solution branding scenarios are available.  No matter which option you choose, you can deploy and register your branding assets on SharePoint sites via the remote-provisioning pattern.
 
+> [!IMPORTANT] 
+> This extensibility option is **only** available for classic SharePoint experiences. You cannot use this option with modern experiences in SharePoint Online, like with communication sites. Notice also that you should **not** take a dependency on the html page structure or on the out-of-the-box CSS style names as these might be adjusted without a notice.
+
 Terminology
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