notebook is a 10 year old macbook pro without macos I installed xubuntu 24.04 in. It comes with an embedded battery.
First notebook I bought, not from apple, had a removable battery. The vendor told me to maximize its life I shouldn't plug the battery in, unless I need it (like for traveling). This way, I've managed to keep the original battery in good working condition for 8 years so far.
Back to the macbook: I cannot remove the battery and constantly loading it to up to 80% and discharge it up to 20% seems ridiculous. Furthermore, this would deplete the battery even faster, I believe.
What can I do to spare the battery as much as possible?
Yeah you don't want the battery to sit at 100% or 0%. I just bulged out my 10 yr old work macbook battery by leaving it plugged in, not the first one i've seen either. There are utils that can set charge limits in linux https://askubuntu.com/questions/34452/how-can-i-limit-battery-charging-to-80-capacity
TBH you MBP battery is probably nearing EOL but it may actually be replaceable.