Sanderson was the 5th overall pick in the 2020 draft and is the second Senator from the 2020 draft to extend long term. His teammate Tim Stutzle is also under contract long term and he went 3rd overall in 2020.
Sanderson is coming off his rookie year where he scored 32 points in 77 games. The Senators are taking a gamble here but if it pays off like Stutzle did, they will be set at defense for a long time.