A slide tackle is performed by a player leaving their feet, and sliding feet first in order to trap or clear the ball in order to deny possession to an opponent.
A slide tackle is legal in soccer. Touching the ball before touching the player is one of the requirements for a legal slide tackle, but is not the only requirement.
Tackling in soccer is an attempt to remove the ball from the player who has it. The ball must be touched, and as long as there is no intent to injure the possessing player, it's a clean tackle....