Assets in setup/gui should now be placed in snap/gui.
introduced in snapcraft 2.26
Originally snapcraft had the idea of putting all fixed assets inside a setup directory, this was introduced with the capability of adding GUI assets such as desktop files and package icons.
With the introduction of the snap directory, the setup directory becomes superfluous and has been deprecated in favor of putting GUI assets in snap/gui instead of setup/gui.
To move to the new schema, all you need to do is:
$ mkdir -p snap
$ mv setup/gui snap/
And if empty:
$ rmdir setup
…and apply those changes to your VCS if you are using one.
See Deprecation notices for further announcements.