277. Work at Home: The prohibition against a kohein with a defect entering the Sanctuary
He shall not come to the curtain, nor approach the altar… (Leviticus 21:23)
The Heichal was the “Sanctuary” of the Temple; kohanim with physical blemishes were not allowed to enter this area. The reason is that they cannot serve, so they have no business there. The Temple isn’t someplace one goes just to hang out. (The kohein with a blemish does have duties he can perform, but these are done in the outer chambers of the Temple.)
This mitzvah applies in Temple times. It is discussed in the Talmud in tractate Bechoros (43a-45b). It is codified in the Mishneh Torah in the sixth chapter of Hilchos Biyas HaMikdash and it is #69 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos. The Ramban (Nachmanides) does not count this as a separate mitzvah.