He didn't have any poor years to be honest. He just happened to miss this year due to injury. The year prior he was the teams leading receiver with almost 700 yards and caught TD's nearly every week.
He was only cut in order to save 10m this year and the Raiders need cap space. At least that's what the reports say.
https://www.pro-football-reference.com/players/W/WillTy00.htm
6'4, 205, 29 years old
2016 - 16 games, 69 rec, 1059 yds, 7 td
2017 - 16 games, 43 rec, 728 yds, 4 td
2018 - 16 games, 41 rec, 653 yds, 5 td
2019 - 14 games, 42 rec, 651 yds, 6 td
2020 - missed due to shoulder injury