The downside is that detecting unsupported browsers require more developing and more maintenance.
Ideally we'd need to perform feature detection for all the modern web API we use to properly warn against any unsupported browser.
My recommendation for forum owners that expect Internet Explorer users to visit their site would be to simply add a conditional IE comment code in the custom header with a message for IE users.
I hope the remaining IE users know that many websites simply don't work with it by now. It's so dead that I would prefer we don't add any IE-specific code in Flarum.
