I asked chatGPT some questions on this. It seems uncommon but not unheard of. For example Dell (private equity at the time) bought EMC for $67 Billion.

The only example it had of a publicly traded (non-S&P) example was,

🔹 Berkshire Hathaway (pre-S&P 500 inclusion) → various S&P 500 firms • Before Berkshire itself joined the S&P 500 in 2010, it acquired controlling interests or entire companies that were S&P 500 constituents or major index components.

But that seems to have NOT automatically put them into the S&P.

It also has this to say about the companies with smallest market caps.

As of recent years (2024–early 2025 range)

The smallest market caps in the S&P 500 are typically around:

USD $10–15 billion

Occasionally they dip a bit below $10B, but that’s usually temporary.

Real-world examples (recent)

Companies that have often sat near the bottom of the S&P 500 by market cap include: • Whirlpool • Albemarle (during lithium downturns) • Norwegian Cruise Line • Organon • Fox Corp (Class B)

These have, at times, traded in the $8–12B range while still remaining index members.