Comment 4 for bug 1925529

Because Impish doesn't appear in meta-release-development yet I had to get a little creative by editing the URI for the meta-release file e.g.:

 $ head /etc/update-manager/meta-release
# default location for the meta-release file

[METARELEASE]
#URI = https://changelogs.ubuntu.com/meta-release
URI = https://bazaar.launchpad.net/~ubuntu-core-dev/meta-release/ubuntu/download/269/meta-release
URI_LTS = https://changelogs.ubuntu.com/meta-release-lts
URI_UNSTABLE_POSTFIX = -development
URI_PROPOSED_POSTFIX = -proposed

Revision 269 had impish in m-r-d. After doing that I was able to verify that the ReleaseAnnouncement is displayed with the version of ubuntu-release-upgrader from -proposed. See the attached screenshot for evidence!