Q:

Where are there discussions about Torah commands that nobody is known to have broken? 

Where are there discussions about Torah commands that nobody is known to have broken?  These discussions can come anywhere, from the Talmud to the Midrashim. 
One example I heard is from Devarim 21:18-21, about a wayward son continues sinning after warning and is found guilty, he is stoned to death by the elders כִּי-יִהְיֶה לְאִישׁ, בֵּן סוֹרֵר וּמוֹרֶה--אֵינֶנּוּ שֹׁמֵעַ, בְּקוֹל אָבִיו וּבְקוֹל אִמּוֹ; וְיִסְּרוּ אֹתוֹ,  ...וּ֠רְגָמֻהוּ כָּל־אַנְשֵׁ֨י עִיר֤וֹ בָֽאֲבָנִים֙ וָמֵ֔ת
If I heard a rabbi's discourse correctly, no son was ever found guilty of this command, really?  That is, the presence of this command alone was enough to scare any son to stay away from breaking this command; whether this rabbi was correct or not (it does not matter, for the sake of this posting), where is there a list of Torah commands (that were discussed by the sages), where there is no record that anybody broke such command, or that nobody was ever convicted or sentenced despite committing the sin because of the Halachic  intricacies of that Commandment? 

A:

There are 3 mitzvot in the Talmud-Gemara Sanhedrin 71a, 1 in Makkos 7a and 1 in Mishna Zavim 2,2 in which the required procedure of punishment have never been enacted by the sanhedrin, due to lack of meeting all the necessary conditions listed according to the tannaim mentioned. 

בן סורר ומורה (Devarim 21,18)- The wayward son(who was so rebellious he is deemed death by stoning early in his life in order to save him from losing his portion in the world to come).  This never happened according to the tanna Rabbi Yehuda because the voice of the son had to sound the same as both the mother and father.  Rabbi Shimon also agrees it never happened, but the tanna Rabbi Yonasan said he sat in front of a grave of a wayward son. 
תניא בן סורר ומורה לא היה ולא עתיד להיות ולמה נכתב דרוש וקבל שכר - The wayward son never happened so why was it taught to get reward for learning about it
עיר הנדחת (Devarim 13,13)- The outcast city (who all worshiped foreign deities and they are deemed death by the sword) This never happened according to the tanna Rabbi Eliezer Since you have to burn every item from the city in a heap and if one house in the entire city had a Mezuza you can't burn the heap or punish the city.  The tanna Rabbi yonasan again argues and said he sat on the mound that was once an outcast city. 
תניא עיר הנדחת לא היתה ולא עתידה להיות ולמה נכתבה דרוש וקבל שכר - The outcast city never happened so why was it taught to get reward for learning about it
בית המנוגע (Vayikra 14, 33) - the plagued house (where there had been tzaraas i.e greenish or redish molds in a house in Israel that renders everything inside the house impure and has a process of scraping the mold off and if it returns eventually knocking the house down) This never happened according to the tanna Rabbi Elazar ben Rabbi Shimon who says that this rule only applies if there are 2 sections of molds the size of a gris (circle with a diameter of 19mm) on 2 different stones on 2 different walls in the corner of the house.  However Rabbi Eliezer bar Rabbi Tzaddok and Rabbi Shimon both claimed to have been to a place where a plagued house was knocked down. 
תניא בית המנוגע לא היה ולא עתיד להיות ולמה נכתב דרוש וקבל שכר - The plagued house never happened so why was it taught to get reward for learning about it
Makos 7a:  רבי טרפון ורבי עקיבא אומרים אילו היינו בסנהדרין לא נהרג אדם מעולם Rabbi Tarphon and Rabbi Akiva said if we were in the Sanhedrin we would never have killed anyone through capital punishment (stoning, burning chopping head of and strangulation).  
But this was not accepted by The rabbanan as the gemoro says היכי הוו עבדי רבי יוחנן ורבי אלעזר דאמרי תרוייהו ראיתם טריפה הרג שלם הרג אמר רב אשי אם תמצא לומר שלם הוה דלמא במקום סייף נקב הוה בבועל את הערוה היכי הוו עבדי אביי ורבא דאמרי תרוייהו ראיתם כמכחול בשפופרת ורבנן היכי דיינו כשמואל דאמר שמואל במנאפים משיראו כמנאפים: How would they have acted (to not award capital punishment)?  They would ask "did you see someone who was going to die within a year kill a normal person" (a tereifa cannot be deemed to death), maybe the sword hit the killed into a wound that was already there and he was a treifa also did you se the adultery the applier of makeup entering into the tube?  Since the eidim cannot testify If they didn't see everything required we can't give capital punishment. 
The Rabbanan however allow the the eidus as long as you see the man and woman together like adulterers i.e even though they don't see exact details but they are sure what happened we use this testimony to punish by death.  
Mishna Zavim (2,2) according to Rabbi Akiva's view
בשבעה דרכים בודקין את הזב עד שלא נזקק לזיבה, במאכל, במשתה, ובמשא, בקפיצה, בחלי, ובמראה, ובהרהור.- 7 There are 7 Ways we check a male zav (on his second white fluid emission that would result in him becoming a fully fledged zav that needs to wait 7 clean days and Ritually contaminates beds and seats etc.)  through excess Food, drink, carrying heavy loads, jumping suddenly, malady, seeing inappropriate sights, thinking inappropriately (these do not make him a zav). 
רבי עקיבא אומר, אפלו אכל כל מאכל בין רע בין יפה, ושתה כל משקה. 
אמרו לו, אין כאן זבין מעתה. 
אמר להם, אין אחריות זבים עליכם. 
Rabbi akiva says even one who eats food whether good or bad or drank any drink (he is not a zav), The sages said to him "in that case there are no zavim at all,"
He said back to them "it is not your responsibility to find zavim." 

Bottom line 3  nonpunishable Mitzvos (lo taaseh): Wayword son, Outcast city and liable to death penalties, and 2 non attainable Mitzvos (and accompanying procedures) , Tzaraas of the house and zav, All subject to opinions of certain Sages. 

A:

I think you are looking for this
Sanhedrin 71a

There has never been a stubborn and rebellious son and there will never be one in the future,...
There has never been an idolatrous city and there will never be one in the future,...
There has never been a house afflicted with leprosy of the house and there will never be one in the future.