How bad is to keep my MacBook plugged in all the time?

Because the portable Mac battery will not be allowed to over-charge

there is no need when near the grid, to un-plug the power adapter.

Save the battery life, by letting the automatic power regulation of

Apple charger work naturally with the MacBook; with fewer cycles.

• Get help with your Mac notebook battery - Apple Support

https://support.apple.com/en-us/HT204054

Since the portable Macs are considered battery-assisted, not battery powered;

safe to say if you prefer to not wear out the battery prematurely, plug into AC.

Charge-cycles would not be accumulating unnecessarily when left plugged in.

Generally newer batteries like those in MacBooks the lifetime works on cycles. so full to empty and back to full charge is 1 cycle.

https://support.apple.com/en-gb/guide/mac-help/mh20865/mac here is a good article how to check battery health but best practice is to keep it between 20 and 85 % charge for long term health. but really as long as you're not draining it to 0 and charging it back to 100 everyday it should last you a long while.

As for you 2015 MacBook, Its likely seen a lot of use so its health won't be 100% as they degrade over time. I'd say to follow guide in the second link to get a bit more life out of it yet :)

here is another useful article about the battery technology: https://www.apple.com/uk/batteries/why-lithium-ion/

matiasheluani wrote:

... which also keeps charging the battery all the time and I was wondering how bad this is.

...

Once the battery is charged completely, charging is put on standby and the MacBook Air gets its power from the adapter.