The original study was with a 50/50 portfolio of large-cap US stocks and US treasuries. More recently, Bengen added internaitonal equities, small-cap stocks, etc. and increased his safe withdrawal rate (SWR).
On the flip side, Morningstar research showed that the projected SWR is below 4% in three of the last four years. The most recent - 2024 - report says the SWR is now 3.7%.