Where are the SELECTION BOXES on Outlook? Why are they NOW GONE???

This appears to be related to a recent change in the message list interface. A setting is being introduced in Outlook.com under Settings > Mail > Message list > Layout, where users can choose between Sender images, Checkboxes, and Name only. To restore the selection checkboxes, open Outlook.com > Settings (gear icon) > Mail > Message list, then scroll down to Layout > How do you want messages to appear in the message list? and select Checkboxes instead of Sender images or Name only. This should display checkboxes beside individual messages and, in supported layouts, the checkbox near the folder name for selecting multiple emails.

Alternatively, if you have Sender images enabled and prefer keeping the profile pictures visible, move your mouse pointer over the sender's profile picture or initials beside an email. The picture should temporarily change into a checkbox that you can select. Once you select a message, Outlook should enter selection mode, allowing you to select additional messages using their checkboxes. This is another way to select multiple emails without permanently changing your message list layout.

If you prefer the previous behavior, I recommend submitting feedback through Help > Feedback in Outlook.com, as there is currently no confirmed setting to restore that exact button.