This is one of those soundbite generalizations to try to explain the world that aren’t really based on anything except anecdotal evidence. and it has a lot of buzzwords without explanation.

The current spate of IPOs for tech companies is being studied to see what the real world effect on many new millionaires in San Francisco. that does not sound naything like a barrier to social mobility.

140 years ago in New York, one could not move up in society unless you were one of The 400, the four hundred “accepted” society families. Social mobility was much more difficult then.

I don’t know anyone that truly cares about social mobility or moving up the social ladder.

Lord, I do. ^^^^

What I have seen a few economists say is that it is not the bottom rungs of the ladder that are being removed, but the ones in the middle. The middle class is disappearing. We are splitting between those on the bottom and those on the top. At least part of the problem is due to automation.
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