Feature request

Is your feature request related to a problem? Please describe.

The modern mode experiment hasn't been maintained in a while and often is the cause of bug reports from users. We should remove this experiment for now until it's reimplemented in the future.

Describe the solution you'd like

Delete all the code related to modern mode.

Describe alternatives you've considered

N/A

Additional context

This feature did not significantly improve bundle sizes, and we made way more bundle size improvements with other initiatives. We'll revisit this once we've hit all the low-hanging fruit of bundle savings.