Has it been mentioned, yet, rules regarding CCL qualifying teams from MLS just changed. Starting next year, only MLS Cup and US Open Cup champs will qualify. No more Supporters Shield winners.
This is just for the 2019 tournament which needs to catch up on two years of MLS qualifying teams. Makes sense to have the trophy winners from each year qualify rather than arbitrarily skip a year. Details from a Sounders website:
For the 2019 edition of the tournament, US Soccer and MLS have decided to only take the team(s) that won MLS Cup and the US Open Cup in 2017 and 2018 to keep it at four teams for the 2019 tournament.
Both US Soccer and MLS intend to return to the old way for the 2020 edition by having the 2019 MLS Cup, Open Cup, Shield winners, and first place team from the other conference qualify.