There are multiple pokemon decompilations that have been around for years which people are using to make their own fan games with a lot of popularity. Some streamers of these games are even invited to official events so it seems like it not necessarily. Any attempt to monetize it usually gets a project shut down though.
I wish it was legal though. This game has been released more than two whole decades ago. Those who spent their childhood playing it now have kids, or maybe even grandchildren! Many of us won't live to see the day its copyright finally expires and that's totally bonkers.
A significant portion of games from that era are probably effectively lost at this point. The current system can't be allowed to continue.