Halakhah zu Schemot 28:3

Thus the abolition of the first claim that says that it is impossible for us to return and fulfill the Mitzvah of Techelet because the Hillazon was hidden is evident since from many examples it is indicated that it was not hidden at all, and that long after the Destruction during the days of the Tanaim and Amoraim, it was available to them, and it is possible indeed almost to the point of certainty, that Maimonides, of blessed memory, also had Techelet. And only afterwards was it forsaken among us due to the exile, which in our many iniquities has increased. And in the Eastern countries it became forbidden for anyone except Royal Nobility as it is written (Esther 8) "And Mordecai went forth from the presence of the King in Royal apparel, Techelet and white, etc." And perhaps Techelet was also forbidden to be placed in Tzitzith, as the lower officials were apt to make additions and be vigilant in all things. Later on I found corroboration to this in Nachmonides' commentary on the Torah (Exodus 28:3)