I think it depends by the manufacturer and how they programmed the charger, I got two HP ProBook that almost exclusively got used when plugged in and after a decade they can still last over two hours, instead my Thinkpad t42 with the same usage pattern now just barely holds enough for shutting down the system
The “built-in UPS” is gonna degrade real quick, based on my experience
I think it depends by the manufacturer and how they programmed the charger, I got two HP ProBook that almost exclusively got used when plugged in and after a decade they can still last over two hours, instead my Thinkpad t42 with the same usage pattern now just barely holds enough for shutting down the system