install: deprecated SDL2

Author: vindarel

content/en/installation/sdl2/macos.md

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+> **Note:** the SDL2 UI is deprecated in favor of the Webview one. It won't receive updates.
+
 ## Using the binary
 
 [Lem has nightly AppImage releases](https://github.com/lem-project/lem/releases)! for Linux, MacOS and Windows.

content/en/installation/sdl2/windows.md

@@ -3,8 +3,11 @@ title: Windows
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+
 [Lem has nightly AppImage releases](https://github.com/lem-project/lem/releases)! for Linux, MacOS and Windows.
 
+> **Note:** the SDL2 UI is deprecated in favor of the Webview one. It won't receive updates.
+
 #### Keyboard Layout
 
 If your keyboard is a JIS layout, you need to put the following settings in `$HOME/.lem/init.lisp`