FYI: Below is a table we published last September showing the S&P 500 stocks that were impacted by the S&P GICS sector re-classifications. Twenty stocks in the index switched sectors at the time, while the three Telecom stocks remained in what became the new Communication Services sector.
Since a number of the stocks involved in the sector re-classifications were mega-caps, the switch ended up altering sector weightings in the S&P 500 quite a bit. Both Alphabet (GOOGL) and Facebook (FB) came out of the Technology sector and went into the Communication Services sector, which lowered the Tech sector’s market cap by $1.3 billion at the time. Other stocks like Disney (DIS), Netflix (NFLX), and Comcast (CMCSA) moved nearly $500 billion in market cap from the Consumer Discretionary sector to the Communication Services sector.
The sector re-classifications last September came at a time when the Tech sector’s weighting in the S&P 500 had ballooned up to 26% of the index. Not since the Dot Com bubble of the late 90s had Tech’s weighting been that high, so the timing of last year’s sector moves was very convenient. The re-classifications basically clipped 5 percentage points off of Tech’s weighting in the S&P, which made things look a lot less lopsided.
So what would the Tech sector’s weighting look like now if the sector re-classifications had not taken place last September? We answer that question in the chart below. Had the re-classifications not taken place, Tech would currently make up 26.53% of the S&P 500. That would be a new high for the current expansion. And the only time Tech’s weighting has been higher than this level was during a 10-month window between December 1999 and October 2000. That turned out to be quite a disastrous time to be long Tech.
Below is a chart showing current S&P 500 sector weightings based on both the old and new sector classifications. As shown, instead of 26.53% based on the old classifications, Tech has a much more digestible 21.14% weighting in the S&P.
Health Care ranks second with a weighting of 14.56%, followed by Financials at 12.64%. Communication Services ranks 4th at 10.24%, and then Consumer Discretionary, Industrials, and Consumer Staples rank 5th through 7th. Energy, Utilities, Real Estate, and Materials combine for a weighting of about 14.5%, which is the same size as the Health Care sector on its own.
These numbers really give you a good sense of which sectors impact movements in the S&P the most, and which ones have basically no impact at all anymore.
Regards,
Ted
https://www.bespokepremium.com/think-big-blog/sp-500-sector-weightings-tech-treks-higher/ 
