If you mean that IE6 represents some common set of baseline features that all other browsers will support this is absolutely 100% false. IE6 has tons of quirks

If you mean that IE6 represents some common set of baseline features that all other browsers will support this is absolutely 100% false. IE6 has tons of quirks and nonstandard features, and sites that are truly written specifically for IE6 and target its features (e.g. activex) and numerous css bugs will simply not work or display correctly in any other browsers at all. Newer versions of IE have to have two separate rendering modes. One of these, "quirks mode," is a non-standard mode that specifically emulates the broken IE6 functionality, because this is the only way to correctly display pages designed for IE6 in a modern browser. On the other hand, people writing new sites in 2021 that support IE are people who are very concerned with compatibility who are being very careful and using zillions of LOC of pollyfills to ensure the sites work in IE. The sites these people create will work in other browsers not because of any property of IE6, but because the people making them also put a lot of work into ensuring they are compatible with other browsers, and dropping support for IE would save a lot of work without reducing compatibility with other browsers.