Return CocoBasic shortcode generator Icon
With WordPress 5.X we have disabled the shortcode generator icon - this icon (add-on) was created for the old editor. To return again this icon, you need to install the Classic Editor plugin - https://wordpress.org/plugins/classic-editor/ and to follow the steps 1-5 from this image:
And add "//" on lines 19 and 21.
Will my theme work without this changes (without shortcode generator icon)?
Yes, your theme will work and all shortcodes will work fine. The shortcode generator just "write" shortcodes (text) for you in the editor and has nothing to do with shortcode functionality.