Twitter has officially shut down its desktop legacy theme today, June 1st, 2020. All users will soon be forced to use the new theme regardless of the browser being used. Twitter began rolling out a new site theme in July 2019, which many found to be a worse user interface than the legacy template. For those who didn’t want to switch, it was discovered that you could continue using the old legacy Twitter theme by impersonating an unsupported browser like Internet Explorer 11 or Twitter’s new theme.
Source: https://www.bleepingcomputer.com/news/technology/twitter-shuts-down-legacy-site-and-forces-new-theme-on-everyone/