phizzy here is one source. This encyclopaedia even indicates the existence of a teacher teaching the pronunciation of the name to his disciples. And even if the name was forbidden to be said aloud after the high priest mentioned died this wouldn't necessarily have stopped people using it and that they did is indicated by evidence from inscriptions, from hymns and magic chants.
The name Yhwh is considered as the Name proper; it was known in the earliest rabbinical works simply as the Name; also as Shem ha-Meyuḥad ("the Extraordinary Name"; Sifre, Num. 143); as Shem ha-Meforash ("the Distinguished Name"; Yoma vi. 2); as Shem ben Arba' Otiyyot ("the Tetragrammaton" or "the Quadriliteral Name"; Ḳid. 71a); and as Yod He Waw He (spelling the letters of Yhwh). The pronunciation of the written Name was used only by the priests in the Temple when blessing the people (Num. vi. 22-27); outside the Temple they used the title "Adonai" (Soṭah vii. 6; p. 38a). The high priest mentioned the Name on Yom Kippur ten times (Tosef., Yoma, ii.; 39b). R. Johanan said the sages delivered to their disciples the key to the Name once in every Sabbatical year. The sages quoted, "This is my name for ever, and this is my memorial unto all generations" (Ex. iii. 15). Here the word "le-'olam" (forever) is written defectively, being without the "waw" for the vowel "o," which renders the reading "le-'allem" (to conceal; Ḳid. 71a). See Shem ha-Meforash.
The restriction upon communicating the Name proper probably originated in Oriental etiquette; in the East even a teacher was not called by name. For naming his master Elisha, Gehazi was punished with leprosy (II Kings viii. 5; Sanh. 100a). After the death of the high priest Simeon the Righteous, forty years prior to the destruction of the Temple, the priests ceased to pronounce the Name (Yoma39b). From that time the pronunciation of the Name was prohibited. "Whoever pronounces the Name forfeits his portion in the future world" (Sanh. xi. 1). Hananiah ben Ṭeradion was punished for teaching his disciples the pronunciation of the Name ('Ab. Zarah 17b). It appears that a majority of the priests in the last days of the Temple were unworthy to pronounce the Name, and a combination of the letters or of the equivalents of the letters constituting the Name was employed by the priests in the Temple. Thus the Twelve-Lettered Name was substituted, which, a baraita says, was at first taught to every priest; but with the increase of the number of licentious priests the Name was revealed only to the pious ones, who "swallowed" its pronunciation while the other priests were chanting. Another combination, the Forty-two-Lettered Name, Rab says, was taught only to whomever was known to be of good character and disposition, temperate, and in the prime of life (Ḳid. 71a; comp. Rashi to 'Ab. Zarah 17b). Maimonides, in his "Moreh," thinks that these names were perhaps composed of several other divine names