They could easily have declared TeamId as non-nullable in modern C# too.

The issue they had is:

(a) TeamId is legitimately nullable at creation, there are valid SlackEvents without a TeamId

(b) They wanted to assert "TeamId won't be null" from a certain point in the code path

(c) They didn't want to create a different data structure (TeamSlackEvent or something) to hold the definitely-has-a-TeamId events.

I strongly suspect (c) was a mistake and they were just too lazy to define a new (sub)-class or record.