What you describe does sound just like a battery that was almost always fully charged, and is about .. six? years old. You may want to try the “rumored to work” calibration process, or replace the battery after the latest OS update:
Which is essentially “set it never to sleep automatically, plug it in overnight, unplug it for 24 hours lid open and let it die and stay dead a while, charge it fully” without ever interrupting the charging process.
On a healthy battery this will slightly reduce capacity each time due to deep cycle so don’t go doing this if you’re not experiencing “shuts down at 42%“ problems!
https://help.ifixit.com/article/265-battery-calibration
Which is essentially “set it never to sleep automatically, plug it in overnight, unplug it for 24 hours lid open and let it die and stay dead a while, charge it fully” without ever interrupting the charging process.
On a healthy battery this will slightly reduce capacity each time due to deep cycle so don’t go doing this if you’re not experiencing “shuts down at 42%“ problems!