Did the phenomenon of "maggidim" (celestial/angelic mentors) who come to teach a person kabbalistic mysteries and other revelations, but begin with those who studied Lurianic kaballah in the 1500's - 1600's or was it always a part of Jewish history after prophecy ended? We have written records of many kabbalists in the 1500's - 1600's and on,being visited by maggidim such as Rav Yosef Karo of Maggid Meisharim fame and the lesser known Rav Shimshon Pesach of Ostropol as well as many others. Was that the beginning of this phenomenon?
The idea of celestial/angelic mentors beings revealing esoteric knowledge goes back to the Talmud and did not start in the 16th century. For example, Elijah reveals esoteric knowledge. In Talmud, Elijah appears as a supernatural being, associated with other angels. See for example Berakhot 4b:
It was taught in a Tosefta: Michael, as stated above, in one flight; Gabriel, in two flights; Elijah, in four flights; and the Angel of Death, in eight flights. (Sefaria).
Fairly early sources such as Pirkei D'rabbi Eliezer (ch. 28) also refer to Elijah as "the angel of the covenant":
שנקרא אליהו ז"ל מלאך הברית
In Ta'anit (22a) Elijah reveals which people will merit entry to olam haba.
ר' ברוקא חוזאה הוה שכיח בשוקא דבי לפט הוה שכיח אליהו גביה א"ל איכא בהאי שוקא בר עלמא דאתי א"ל לא אדהכי והכי חזא לההוא גברא דהוה סיים מסאני אוכמי ולא רמי חוטא דתכלתא בגלימיה א"ל האי בר עלמא דאתי הוא
Rabbi Beroka Ḥoza’a was often found in the market of Bei Lefet, and Elijah the Prophet would often appear to him. Once Rabbi Beroka said to Elijah: Of all the people who come here, is there anyone in this market worthy of the World-to-Come? He said to him: No. In the meantime, Rabbi Beroka saw a man who was wearing black shoes, contrary to Jewish custom, and who did not place the sky-blue, dyed thread of ritual fringes on his garment. Elijah said to Rabbi Beroka: That man is worthy of the World-to-Come. (Translation from Sefaria).
Yoma (19b-20a) similarly describes Elijah transmitting esoteric information:
דא"ל אליהו לרב יהודה אחוה דרב סלא חסידא אמריתו אמאי לא אתי משיח והא האידנא יומא דכיפורי הוא ואבעול כמה בתולתא בנהרדעא אמר ליה הקב"ה מאי אמר אמר ליה (בראשית ד, ז) לפתח חטאת רובץ ושטן מאי אמר א"ל שטן ביומא דכיפורי לית ליה רשותא לאסטוני
Elijah the Prophet said to Rav Yehuda, brother of Rav Salla Ḥasida: You have said and wondered: Why has the Messiah not come? Why is that surprising? Isn’t today Yom Kippur, and relations were had with several virgins in Neharde’a, as the men and women stayed awake all night and that led to promiscuity? Rav Yehuda said to him: What did the Holy One, Blessed be He, say about those sins committed by the Jewish people? He said: This is what God said: “Sin lies in waiting at the door” (Genesis 4:7), and it is no wonder that men sin. He asked him: And what did Satan the prosecutor say about their sinning? Elijah said to him: Satan, on Yom Kippur, has no license to prosecute (Trans. Sefaria).