hellekin is looking to restore the search icon in the top bar, which has been replaced by a large ‘Welcome’ banner. Moin explains that the search icon is hidden when the welcome banner is visible and directs hellekin to the themeable site settings where the welcome banner and search experience can be adjusted. NateDhaliwal notes that Meta has both the icon in the header and the Welcome Banner, and Moin clarifies that Meta does not use the core banner and that the banner hides the search icon in safe mode.
I think the search icon is hidden when the welcome banner is visible.
The settings to disable the welcome banner and adjust the search experience are now themeable site settings. You find them when you edit your theme at /admin/customize/themes and at /admin/config/theme-site-settings