A reflection of Glasc Mixologist is holding battlefield. The beginning phase begins and reflection goes away. I play the deathknell unit on the battlefield where my reflection was. Do I get a point from hold?

Based on the rules and your scenario, yes, you do get a point from Holding.

While you previously understood that having items on the chain prevents you from losing control, Rule 464.2 confirms that you score from a Hold simply by maintaining control of a battlefield during your Beginning Phase. Because the Deathknell ability creates a pending item on the chain, you maintain control of the battlefield throughout the Beginning Phase, satisfying the requirement to score.

The Reasoning

1. Requirement for Holding: According to Rule 464.2, a player scores from a "Hold" by maintaining control of a battlefield during their Beginning Phase.
2. Chain Protection: Items on the chain prevent you from losing control of a battlefield (Rule 187.4.c).
3. The Sequence:
   • Beginning Phase: The game checks for control to determine if a point is scored.
   • Reflection Leaves: Your Reflection token leaves the battlefield, but because the Deathknell ability is placed on the chain, you do not lose control.
   • Resolution: You retain control because of the pending item on the chain. Since control is maintained throughout the Beginning Phase, the condition for scoring is met regardless of whether a unit is physically present at the exact moment of the check.

Summary

Because the Deathknell ability creates a pending item on the chain, you maintain control of the battlefield during the Beginning Phase. Under Rule 464.2, this is sufficient to score the point from the Hold, even if your previous unit left the battlefield before the new one arrived.

Note: I could not fully verify this ruling against the retrieved rules.