It strikes me that the legal definition of insider trading is very difficult from what we commonly think about in finance/economics.
As someone who has traded index strategies and seen index changes, we always treated that information as material. That means receipt of this information before it was made public would be inside information.
Second, this 2013 interview with the S&P Dow Jones Index Chief, David Blitzer, makes clear that they also view such information as inside information.
Trading on inside information is insider trading. It is also stunningly easy to find, since the exchanges have all the timestamped information with customer information. I would not advise trading on any such information.
Here is what the world knows about inclusion in the S&P500:
To be eligible for S&P 500 index inclusion, a company should be a U.S. company, have a market capitalization of at least USD 8.2 billion, be highly liquid, have a public float of at least 50% of its shares outstanding, and its most recent quarter’s earnings and the sum of its trailing four consecutive quarters’ earnings must be positive.
It is fairly predictable that certain names will soon be included (eg. TSLA now that it has met the earnings goals).
So you can trade on the knowledge that Tesla is going in, and many other people are doing the same.
Any knowledge you have beyond this from an inside source, is insider trading.
I dunno what to say, really.
If it sounds too good to be true, it almost certainly is.
If it just feels wrong, it almost certainly is.
Even in the soundest jurisprudences like the UK and US common law, laws have been post-hoc interpreted to criminalise activities that were not obviously criminal ex-ante.
If it smells right to you; and you are confident worst-case that a dozen randomly-selected people from the same neighbourhood would see it the same way as you, then OK. But their jurors' "smell-test" might significantly differ from yours. If they used your posting above as evidence to suggest you knew how S&P might behave with its ratings, how confident would you be if actually charged in court?
If it was a one-off, and the prosecutor painted you as an "opportunistic one-off" versus you did this many times, and the prosecutor painted you as a "serial manipulator", how would that distinction make you feel about the broader scenario?
[Because one cannot be "inside" on the dollar, Treasury Bonds, or Gold. One cannot be on the "inside" of the S&P500 index... but one can very easily be on the inside of any stock within or without. So if you knew which stocks out were replaced with which stocks in, and knew which way that adjustment would drive the index, no possible crime committed. But if you knew the entries and exits, traded that, then possible jail-time awaits, unless you have a REALLY good excuse/explanation.]