That’s the most ridiculous license statement for a “free” app/plugin that has ever been published.
That’s no exaggeration, that’s the truth.
As we should praise the good ones, we should also criticize the ridiculous ones,
if we are after a better world, that is.
Ask the maintainers nicely to add a license. Unless the software includes strong indications to the contrary, lack of a license is probably an oversight. If the software is hosted on a site like GitHub, open an issue requesting a license and include a link to this site, or if you’re bold and it’s fairly obvious what license is most appropriate, open a pull request to add a license.
Don’t use the software. Find or create an alternative that is under an open source license.