The previous icon was not available anymore. Also the error message gives the user a suggestion on what to do.
IMO this is the best we can do with the little space we have available.
The hspell plugin is also recommended when installing the deb packge, see also https://docs.kde.org/trunk5/en/kde-workspace/kcontrol/spellchecking/index.html.
Another related issue is fixed in frameworks/sonnet!2 (merged).