For anyone using the quantmod library in R, Joshua Ulrich (the package's author) released a workaround that you can get by updating quantmod to the development version. In R, do either:

remotes::install_github("joshuaulrich/quantmod", ref="157_yahoo_502")
or
devtools::install_github("joshuaulrich/quantmod", ref="157_yahoo_502")

More info on the GitHub page:
https://github.com/joshuaulrich/quantmod/issues/157

This works for me. I was able to successfully use quantmod::getSymbols() to query the Yahoo API.