Does anyone know where the famous shittah of the maharam mirottenburg is brought down about him being chosheh for shittas Baal hamaor (who holds you have not fulfilled the mitzvah of tzitzis if you are missing one of the components-either blue or white) on shabbos to not wear tzitzis in reshus harabbim?
The Tur in Orach Chaim 301 says this in his name.
כשמצוייצת כהלכתה לא הוי משוי הלכך מותר לצאת בטלית שלנו לרשות הרבים דחשיבא מצוייצת כהלכתה אע"פ שאין בה תכלת ד קיימא לן אין תכלת מעכבת הלבן תדע לך שהרי אנו מברכין עליהם והר"מ מרוטנבורק היה מחמיר שלא לצאת בה
However, it might not be so clear this was in fact Maharam's opinion.
See footnote 162 in Machon Yerushalaim edition of the Tur where he points to Tashbatz Katan #44 who says Maharam was stringent not to wear tzitzis on Shabbos since he was afraid one of strings would rip. The footnote continues and says Tshuvas Maharam Lvuv edition siman 365 writes two reasons not to wear them, perhaps they are not made properly, and also because there is no Techeles.
The footnote also points to the Bach.
Bach wonders why a fear of a string ripping would matter if the lack of Techeles is an issue. Bach points to the Mordechai in perek Chavis #442 who explains Maharam stringency regarding wearing his talis applied only to his talis katan, since he wouldn't be able to check it in the reshus harabim as it was under his clothes. But he posits Maharam would in fact go out with them if he knew they were fine, even though they didn't have Techeles.