This isn't all entirely true.
Yes, you can set up almost any rim tubeless, but a tubeless specific rim will have a shelf for the tire bead to sit on, this helps keep the tire in place in the event of a flat, you can get the bike slowed and stopped before the tire comes off the rim. (I put several hundred miles on a ghetto tubeless set up on a MTB, the rim wasn't officially tubeless ready, but it did have the bead shelves)
IMO there's no reason to set up a non-tubeless tire tubeless these days. For MTBing pretty much everything is tubeless ready now.
For road use a non-tubeless tire WILL blow off the rim if you set it up tubeless.
For mixed surface use the tire pressures are high enough that a blow off is a real possibility with a non-tubeless tire. I know several folks who've had that exact experience with tires at 35-40psi. For a while I had 38's on my gravel bike, a non-tubeless Compass Barlow Pass, set up tubeless. 45psi seemed to be the right air pressure for me on the rear wheel, and the few hundred miles I put on the set up was solid with no issues, but, I was never comfortable with it. Every downhill, especially if it was rough, I had the fear of a blow out in the back of my mind. I swapped them out for something made to be used tubeless.
You are the one riding the bike, it's your decision what to use. But in my opinion, (and for me personally) if you're going tubeless, stick with tubeless rims and tires. Tire setup is also WAY, way, way, way easier when using parts designed for tubeless.