But Citi started that suddenly (without advance notice) last year, and says that it's because it's a checking account. So I wouldn't trust any other bank's offers for a checking account bonus, because what's to stop them from (in the near future) adding inflated 1099s just as suddenly (and without advance warning) as Citi did?

Citi did this to people in retrospect, ie, people had applied for the checking account with the signup bonus before there was any mention of 1099s, then after people got the bonus, they suddenly started getting 1099s they hadn't been expecting.

Can you really trust that no other bank will do this in retrospect either, if their lawyers look at it and agree with Citi's lawyers that checking account bonuses are subject to taxes?