True, though Buffett did say the 90% S&P500, 10% cash portfolio would be fine for anyone.
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Unless a retiree is so rich they are sure you can live from the dividends alone, Mr Buffett's 90/10 it isn't really an answer to the problem of how to fund one's retirement from a portfolio sensibly.

A random thought on that.

The current dividend yield of the S&P 500 is about 1.40%. Equally weighted (RSP) is a bit higher at 1.64%.
But these days you can buy a whole slate of stocks at many brokers with a single command.
If one were to buy an equally weighted portfolio of only those S&P 500 stocks paying over 1% dividend yield, you'd still have 321 stocks, presumably enough diversification for anybody. Reconstitute it from time to time when you feel like it--annually is often enough, or whenever. The average dividend yield of that portfolio is currently about 2.92%, over twice that of SPY, making withdrawals a heck of a lot easier to decide about. Despite just having had a bad year relative to the S&P 500, this approach has actually performed a bit better over the last 20, 25+ years. In any case, they're pretty close: no obvious reason to think one will do better than the other over time.

An added benefit of owning the stocks directly yourself, other than no management fee, is that you don't have to follow the S&P index component changes the day they do them, a very simple way to guarantee you have an edge over time.
researchaffiliates.com - Revisiting teslas addition to the
researchaffiliates.com - Buy high and sell low with funds
The implementation is simple: do every index change they do, just don't do it right away. Wait a year or so. Outperformance by laziness.

Jim