At the time of this question String.prototype.at is not implemented in all browsers.
I would like to use the at function on the String prototype within a React app that uses babel to transform modern ecmascript into older more browser compatible versions of javascript.
I have been unsuccessful in finding a babel plugin that would do this for me.
Is there a @babel/plugin that will transform the String prototype at function down to more browser compatible ES5 javascript?
Babel is for syntax-level transformations, not browser API polyfills.
You can consider using a polyfill service like Polyfill.io, or alternatively, simply use the polyfill code from the relevant proposal.