Via Doctor of Credit/Reddit, Citi has changed the language on their credit card applications such that you cannot receive a signup bonus again until it has been 24 months since you last opened or closed the same card. Previously, they only required an 18-month wait.
As a reminder, American Express recently ended all churning of their cards, and there has been a unconfirmed rumor floating around that Chase will be limiting all of their credit cards to the 5/24 rule, which denies any credit card application if you’ve opened 5 or more credit cards in the past 24 months.