What kind of maintenance do 3D printers require?

Pretty much just cleaning and lubrication every so often.

Mechanical / drive mechanism parts will wear out eventually, but all you can do is minimise wear due to dirt and lack of lubrication, then replace the parts when they wear too much.

I've had mine several years and the only thing I've had to replace so far is the heater cartridge in the extruder.

They require maintenance in 4D.