Batteries have a limited amount of charge/discharge cycles - if you are doing this you're unnecessarily wasting those. Keeping the device plugged in once charged is absolutely fine - the battery itself will stop charging and the power will be used only to power the device itself so it doesn't have to touch the battery anymore.
This stupid advice has been going around for ages and if it ever applied it definitely doesn't apply now - all modern lithium batteries use charging circuitry that stop charging beyond a certain point regardless of whether it's plugged in or not - if they didn't you'd be seeing battery fires everywhere.
Macs do this automatically but it doesn't seem to happen to me that often. Usually if I leave it plugged in for an entire weekend I think.
Two reasons - One, the charger heats up a lot when I keep it on. Of course, you don't feel it unless you are nearby or touch it. Two, in the past I once had the charger destroyed due to a power surge. The Macbook remained fine after that power surge but I had to replace the charger since it stopped charging after that.
I have three of them plugged in 24/7 for 3 years now and recommended them to others, all houses still standing. But you can also get the original 96 W charger from Apple, it's bigger so it can dissipate heat better than the smaller ones.