Looking at for example 1965, the top 10 of the S&P500 were, rounded, ATT 9%, GM 7%, Exxon (4%) IBM (4%) DuPont (3%) Texaco (3%) Sears (3%) GE 2%, Kodak 2%, Gulf 1%, for a total of 38%.

This is pretty close to current concentration, but the current top 10% is basically all technology except for Eli Lily at 1.5%, so in that sense it's arguably unprecedented.

There's a good chart here on page 5 of top 10 weights over time, and on 6 of how the 1965 top 10 fared to 2025.

https://corporate.vanguard.com/content/dam/corp/research/pdf...