You also have the option to use course import. In each course that you want the announcement to be in, go to settings. The right-side menu includes "import course content," and when your import type is "copy a canvas course," you have an option to "select specific content." The content list will allow you to select the specific announcement you want to copy. This is only really an advantage if you are copying a group of announcements to each course, otherwise copying and pasting is faster.
You might appreciate an idea conversation about adding the option to post announcements to multiple courses: https://community.canvaslms.com/t5/Idea-Conversations/Common-announcement-to-all-my-classes/idi-p/37...
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