Isn't it time we stopped regarding vttest as the arbiter of terminal "emulator" correctness? It's in many ways an impressive and useful program, but its existence has caused much programming effort being wasted in the pursuit of a perfect vttest "score" — be it VT-52 mode, DEC line drawing characters, tab stop setting, double-wide characters, answerback messages, and other useless features. (I know, because I have been one of those programmers. I'm better now, thank you.)
Some of these "mandated" features are actually negative, as they make it too easy to get the terminal into a broken state, or can be outright security holes.