In Moodle 4 and higher the "Custom menu items" (custommenuitems) setting in the "Theme settings" no longer renders tooltip titles on the page on mouse hover, contrary to this help description (my bold):

"A custom menu may be configured here. Enter each menu item on a new line with format: menu text, a link URL (optional, not for a top menu item with sub-items), a tooltip title (optional) and a language code or comma-separated list of codes (optional, for displaying the line to users of the specified language only), separated by pipe characters."

I tested the same menu items in Moodle 3.11, then in Moodle 4.0.* and 4.2.1 (Moodle Cloud), using the Boost theme. The titles do show in Moodle 3.11, but they do NOT show in Moodle 4 and up.