Charles Ellicott Commentary 2 Kings 23:9

"Nevertheless the priests of the high places came not up to the altar of Jehovah in Jerusalem, but they did eat unleavened bread among their brethren." — 2 Kings 23:9 (ASV)

Nevertheless ... came not up to the altar. — Only the priests of the high places did not offer at the altar. They were not permitted to do so, being considered incapacitated for that office by their former illegal ministrations.

But they did eat. — They were not even allowed to eat their share of the meat offerings with the legitimate priests; but had to take their meals apart, among their brethren, that is, in their own company. (Leviticus 21:21–22.)

Eat of the unleavened bread. — Omit of the. The phrase is a technical one, meaning to live upon offerings. (Leviticus 6:16–18; Leviticus 10:12.) These irregular priests were probably employed in the inferior duties of the Temple.