The issue is that many libraries such as react-scripts, have not updated from PostCSS 7 to PostCSS 8.
I also did a forced update of some of my dependencies for the same reasons, and during the React build, saw this message:
Post-css-modules-values: postcss.plugin was deprecated. Migration guide:
https://evilmartians.com/chronicles/postcss-8-plugin-migration
Identical messages mentioned all of the following as well, referencing the migration guide:
postcss-modules-local-by-default, modules-extract-imports, postcss-modules-scope, postcss-import-parser, postcss-icss-parser, postcss-url-parser, postcss-flexbugs-fixes, autoprefixer, postcss-ttribute-case-insensitive, css-blank-pseudo, postcss-color-functional-notation, postcss-color-gray, postcss-color-hex-alpha, postcss-color-mod-function, postcss-color-rebeccapurple, postcss-custom-media,
postcss-custom-properties, postcss-custom-selectors, postcss-dir-pseudo-class, postcss-double-position-gradients, postcss-env-fn, postcss-focus-visible, postcss-focus-within, postcss-font-variant, postcss-gap-properties, css-has-pseudo, postcss-image-set-function
Then I got an error (almost identical to the op's), involving Bootstrap and PostCSS:
./node_modules/bootstrap/dist/css/bootstrap.min.css (./node_modules/css-loader/dist/cjs.js??ref--5-oneOf-4-1!./node_modules/postcss-loader/src??postcss!./node_modules/bootstrap/dist/css/bootstrap.min.css)
TypeError: Cannot read property 'unprefixed' of undefined
Many packages include PostCSS as a dependency.
Looking at the docs about updating plugins for using PostCSS 8.0 it seems that waiting until packages in our projects update to the PostCSS 8.0 API, or manually forking and editing them per PostCSS's 8.0 update documentation, may be the only options for some projects.
A better answer than this would be appreciated.