That was already the case this year but they never acknowledged it, now Google is putting the final nail in the coffin as it relates to the “first Monday of the month” updates:

We also dug into how we can deliver the highest quality, best tested updates to Pixel users on a consistent basis. As part of this effort, our security updates, bug fixes and feature updates won’t roll out on a specific day each month. Instead, we’ll deploy updates as soon as they’ve completed the necessary tests to ensure they improve the experience for all Pixel customers.