Is it forbidden to shave off the sideburns and extend them to the cheek or past the earlobe during a haircut, knowing that this is purely for cosmetic purposes?

Shaving the beard is not only contrary to the Sunnah, but it is also forbidden according to the majority of scholars. Among its benefits are adornment, dignity, awe, and distinguishing between men and women. If shaving it is a characteristic of the polytheists, then that increases the sin. You must fear Allah and obey Him.