Transpiling your code is not quite enough to ensure legacy compatibility. Polyfills are the second step to ensuring legacy support.
Here are the recommended polyfills:
core-jsto polyfill core/common native features:
Object.assign, array methods, and others.
regenerator-runtimeto polyfill native generator functions.
whatwg-fetchto polyfill native
fast-text-encodingto polyfill native
NPM install example:
npm install core-js regenerator-runtime whatwg-fetch fast-text-encoding