Besides the few surviving designs based on much earlier work there has been nothing new in the massless speaker world that has made it past the theoretical or prototype level in over the last 20 years. That's not to say it has been a stagnant area with many attempts being made to take products to market, a lot of research and patents galore.
One notable rise in the patent world is in Asia, especially the number of Chinese patents around massless speakers has ballooned. Having spent considerable time wading through many of them it looks on the face of it that most if not all are patenting designs that have been patented or open long ago in the western continents. They are difficult to search and understand for someone like me from the English speaking world so perhaps there are some nuggets or gems in there that I have yet to find.
Notable more recent patents include the corona wind wire based speaker of Howard 1997, Sony's own plasma tweeter application 2008 and a microwave based plasma tweeter (can't see anything to go wrong there) from Wenjie 2018.