Most third party apps that have menu bar items can also optionally hide the items. For example, the volume item can be restored by turning on “Show volume in menu bar” in the Sound preferences. If you ever need to re-add the item, there’s typically a check box in the related System Preferences pref pane.
If you don’t want to see an item, holding command (⌘) and dragging it out of your menu bar should remove it.
Most system menu bar items can be removed. If your menu bar is full, like the image below, additional items will not show. Status items never appear on the left side of the notch. Any items that can not fit on the right side of the notch will be hidden by macOS. The camera housing at the top of the M2 MacBook Airs, 14-inch MacBook Pros, and newer 16-inch MacBook Pros can mean there is reduced space for menus and status items. In this situation, iStat Menus is running perfectly, but hidden from view. This is because the menu bar is full, and can not fit other items. Menu bar items not showing On smaller screens, and in situations where there’s a lot of menu bar items, macOS may hide some items.