Prohibition of lying

It's not clear to me where you get this "simplicity", but most poskim do not believe so (not just Rabbeinu Yonah). It is also quite explicit in the Shulchan (Yod Tev Tev Si B).

Taking this from simplicity. “Speaking falsely, stay away” is not among the main commandments. Even those who wrote that there is a prohibition, some qualified it only when the lie causes harm (the question dealt with lying without harm).
See a brief overview here: https://www.google.com/amp/s/www.kikar.co.il/_amp/parshas/379116

And regarding the answer, I don't know what you meant. There is no such sign in Judaism, and there is no such section abroad either.

All its simplicity. When I read the verse, I don't see any hint that it is specifically a lie that causes harm (so I demand that you do not kill the innocent and the righteous), but that lying is something that should be avoided.
That article is very partial, it is recommended to consult the book Niv Sfatayim, for example, for an in-depth discussion. Even here https://www.hamichlol.org.il/%D7%90%D7%99%D7%A1%D7%95%D7%A8_%D7%A9%D7%A7%D7%A8 you can find a more comprehensive discussion.
Some of the lists of the commandments do list (עיהן), and in any case it is not necessarily any kind of index.
I don't know which edition you have, mine appears there. For the sake of convenience, I will quote: "If someone dies and is not known to him, it is not obligatory for them to tell him, even his father and mother. And this is why it is said: He who spreads slander is a fool, and it is permissible to invite him to an engagement and wedding feast, and any joyous occasion, since he does not know who he is. If asked about him, he should not lie and say, 'He is alive,' as it is said: 'Keep away from a liar.'"

I use the Shulchan project. For some reason it didn't scroll down for me and stopped at S.A. or B. Strange.
In any case, the source of this halacha is a matter from the 2nd chapter of the book of the Law, and it is very far from learning halacha from there. Especially since the Gemara itself learns three things from there and this is not one of them. Therefore, from the Gemara's perspective, it looks at most like good conduct. As is known, the Shulchan does not make a careful distinction between halacha and good conduct.