Back at Microsoft's Build 2019 developer conference in May, Microsoft announced IE Mode for its new Chromium-based Edge browser. The idea was that you'd be able to open a page in Internet Explorer, but within Edge in a new tab. The feature was finally activated a couple of weeks ago ahead of Microsoft's Inspire partner conference.
You can enable a flag to do it, and previously, it would show an option in the menu called 'Show this page using Internet Explorer'. That menu option has since disappeared, even if you still have the flag enabled.
As spotted by Techdows, the reason is because IE Mode is meant to be an enterprise feature, and the option to manually launch a site in an IE tab was only meant for "debugging purposes". Instead, IT admins will be able to control when a certain site will launch in an IE tab, so it will just happen automatically.
To be fair, this isn't a change that will affect consumers. The actual need to switch from Edge to Internet Explorer isn't something that is common outside of the business world. While IE Mode was probably a fun toy to play with for a little while, users should be fine without it. And if you really miss it, Internet Explorer is still installed on all Windows 10 PCs.