What needs to be fixed

CMSApplication
The $_clientId, $_messageQueue and $_name class properties have all been renamed to remove their underscore prefix.
Now implements CMSApplicationInterface
If there is an error retrieving a Pathway, Router or Menu object using the respective methods the exception will now bubble up rather than the method silently catching the exception and returning null
CMSApplication::getInstance will now additionally try and load the user object (using the loadIdentity function)
CMSApplication::isSite and CMSApplication::isAdmin have been removed as with the addition of the ConsoleApplication and ApiApplication can give misleading results. Please use CMSApplication::isClient (available from Joomla 3.7). For more information please see Discover on which client your extension code is running

Why this should be fixed

Still present into Joomla 4

How would you fix it

grep -iRl "isAdmin" /home/public_html

/public_html/layouts/joomla/edit/admin_modules.php
/public_html/libraries/src/Form/Field/ListField.php
/public_html/libraries/src/Helper/ModuleHelper.php
/public_html/administrator/components/com_menus/Controller/DisplayController.php
/public_html/administrator/components/com_modules/View/Modules/HtmlView.php
/public_html/administrator/components/com_modules/tmpl/modules/default.php
/public_html/administrator/components/com_modules/tmpl/modules/default_batch_body.php
/public_html/administrator/components/com_modules/Model/ModuleModel.php
/public_html/administrator/components/com_modules/Controller/DisplayController.php

Side Effects expected