I think learning on a 22lr is the way to go for anyone who has little to no firearms experience.
But... a person can learn on a 9mm or larger, its just that the learning curve is longer. A 22lr can help you learn the fundamentals much quicker.
It is also dependent on the person in question that is learning, some can handle starting on a centerfire better than others. I have seen people at the range, struggling with a 9mm, I hand them a 22 and groups immediately shrink.
22lr ammo can be harder to come by, but the stock is building back, at least where I am at. Prices are still higher, but it sits on the shelf for days now... Though, if the price is low, the stock does go quickly. As people begin to realize the supply is there, and that it just needs a little bit of smart buying (read, less rushes) to get the shelves full and prices down, it will get better.
223/556 was hard to come by not long back, but now I am seeing it for prices lower than a couple years ago quite often.
Some say if you start with a 22, you will just sell it to get something bigger... I think that is a little short sighted. As having a 22 in your possession is always a good thing, one should not sell the 22, they should keep it and get a second firearm when ready.
I have been shooting most of my life, and I still love/enjoy a good 22 pistol or rifle. They are fun, and still cheaper to shoot than other options. In general, great target/range guns. Quieter, which is good, though ear plugs are still needed. Great for pest control as well, if you live in the country and need such a thing.
For not a lot of money, you can have a very mechanically accurate 22 pistol. (like the Ruger or Browning) With something like that, any issues in group size can be attributed to the shooter, meaning less doubt on whether it is the gun at fault for poor aim. (this is something many do, blame the gun)