Can I use matte medium to seal a painting if I don’t want the glossy sheen?

Yes, matte medium will give your painting a matte sheen. The sheen of the finished painting will depend only on the final varnish coat. It’s worthwhile for you to experiment to figure out what you like best. Golden brand paint also makes a satin UVLS varnish that has a lovely patina. Understand that a varnish is typically a removable, protective final coat. A thin coat of buffed cold wax, for example, has a lovely, slightly glossy sheen as well.