Having just upgraded the OS, I expected that (like my iPhone and iPad) a low battery warning would appear.
On both of these devices, the percentage shows alongside the icon and the icon changes colour when battery power gets low; however, on my MacBook, there's just the icon top right.
To re-iterate, on my MacBook, not only is there no warning issued but I need to interrogate the battery icon to see what's left, rather than being able to just glance at the icon like on the iPhone & iPad.