> This has been a thing in the CRSP indexes (ie. the benchmark for Vanguard’s VTI) forever.

CRSP has recently changed their rules:

> CRSP indexes were also recently changed to better accommodate fast entry. New IPOs are eligible for CRSP's suite of indexes after five trading days, provided they pass the index's eligibility and investability screens. Previously, these screens included having at least 10% of shares qualifying as freely tradeable (known as float shares outstanding, or FSO). However, in April the methodology changed to allow stocks with either 10% FSO or approximately $3.3 billion in float-adjusted market capitalization to be eligible for index inclusion. The weighting of stocks in CRSP indexes is also based on free float, which should help address the investability challenges associated with thinly traded stocks.

* https://www.schwab.com/learn/story/some-indexes-accelerate-e...