Due to the fact that COMBAT_TEXT_UPDATE is for any event in combat, including DOT ticks. The combat time is updated, it cannot be replaced with UNIT_COMBAT, since it has similar behavior. But the periodic damage itself is tracked via CHAT_MSG_SPELL_PERIODIC_SELF_DAMAGE, which may be important for filtering out such events
Due to the fact that COMBAT_TEXT_UPDATE is for any event in combat, including DOT ticks. The combat time is updated, it cannot be replaced with UNIT_COMBAT, since it has similar behavior. But the periodic damage itself is tracked via CHAT_MSG_SPELL_PERIODIC_SELF_DAMAGE, which may be important for filtering out such events