should we add a undo_ftplugin variable?

maybe also make the iskeyword setting be set in the ftplugin if the vim version is too old

Yes we should. But to make it work one of the following should be done

• provide full filetype plugin which set b:did_ftplugin variable
• move current filetype plugin to after/ftplugin directory

In other cases filetype plugin bundled with Vim will override b:undo_ftplugin.