Does Stupefy or Frigid Touch trigger Immortal Phoenix effect to play it from trash?

No, neither Stupefy nor Frigid Touch will trigger the ability of Immortal Phoenix to be played from your trash.

According to RiftJudge FAQ #2625, Immortal Phoenix only triggers when a unit is killed by a spell (either via a "Kill" instruction or via damage dealt by a spell). Stat reduction effects (reducing Might) do not count as "killing a unit with a spell," even if that reduction eventually causes the unit to die due to the game rules during a cleanup phase.

Why they don't trigger Immortal Phoenix:

• Effect Type: Both Stupefy and Frigid Touch are stat-reduction spells. They reduce Might rather than dealing damage or issuing a "Kill" instruction.
• System Rules: Units are only "killed by a spell" for the purpose of Immortal Phoenix if the spell has a specific "Kill" action or deals damage that causes the unit to die in the immediately following cleanup (see Rule 428.5.c and FAQ #2625).
• Result of Reduction: As clarified in RiftJudge FAQ #8663 and #80, reducing a unit's Might (even to 0 or below) does not constitute a "kill" by the spell. The death is a consequence of system rules checking Might vs. damage during cleanup, not an action attributed to the spell's effect.

This interpretation is supported by RiftJudge FAQ #2625, which explicitly states that Immortal Phoenix cannot be played from the trash when a unit is destroyed by stat reduction effects.