Looks like at least now it's an open ticket (if I'm interpreting https://community.canvaslms.com/t5/Canvas-Ideas/Assignments-Assignment-Bulk-Update-Negative-Date-Shi... correctly).
Changing dates takes days. Why is there a bulk date edit (albeit still dysfunctional) for assignments, but not for announcements, and not for quizzes (that can be ungraded), and not for when modules unlock? Why isn't there a way to map the dates when a course is copied in segments (e.g. one before the Spring Break and one after)? Or at least have a Spring Break feature where you can specify if you are adding or removing one and what the dates are. Spring Breaks are common enough, you would think. It takes a week to set up a semester worth of classes and it's mostly editing dates one by one ad nauseum.
And then for the simplest of things clicks are added, not removed. To change one announcement now, you need to click on it, then click on the ... menu, then click on edit, then get to the bottom of the page, then actually edit the date, then click on save, then click on this novel pop-up to not send an update, then get back to the announcement list. Could we not just display the announcement date chooser next to the announcement even if bulk edit isn't implemented? And while we are at it, a checkbox for enabling or disabling an announcement? Instructure really needs a UX expert or two.
And for a bug like only supporting positive date shifts to stay there for years? Academia is so forgiving.
Okay, done ranting. Looks like they are fixing this one at least.