In late April 2023 Yahoo again changed the API procedure for
downloading quotes. Sometimes the old way works, sometimes it
doesn't. Speculation is that whether it works or not depends on which
yahoo server gets your request.

The previous time, sometime around 2017, you could just download from:
Yahoo Finance: V7 Finance Quote{SYMBS}
Where SYMBS is a comma separated list of symbols.

and to get stock historical price data you had to use curl with --cookie-jar
and parse that to get the CrumbStore (crumb). Then to do the query
you have to use the cookie and append "&crumb=[the crumb]"
But now the cookie-jar is empty and it works with an empty crumb.

So they unprotected historical prices and kinda protected quote data.

To get the quotes you need a crumb.
To get the crumb you have to do "Yahoo Finance: V1 Test Getcrumb"
But to do that, you need the cookie that Yahoo gave you from before????
But doing that in a browser works, I cannot figure out yet how to get that in curl.
Now you need to append "&crumb=$crumb" to the above URL.

Most of the free places that had an API to download stock prices are
defunct or are no longer free.

Tiingo is still up and free, but does not have index data, like
yahoo's ^GSPC for S&P500 index.