COBOL springs to mind. It sounds like you have a profitable niche supporting legacy stuff. Edge came out five years ago and IE11 was depreciated.

You can try the polyfill approach. This is generally not recommended as it wastes bandwidth. But since your customers have bandwidth you could go with polyfills. Or just stay stuck in the past with a niche of writing IE11 compatible stuff. That could be career limiting, IE11 is getting as irrelevant as 8.3 FAT filenames.