If a unit with Guardian Angel dies, does the Deathnell still trigger?

No, the Deathknell ability will not trigger if a unit with Guardian Angel dies.

Reasoning:

• Replacement Effect: According to RiftJudge FAQ #8433 and Rule 369.1, Guardian Angel acts as a replacement effect. When the unit would die, the effect replaces that "death" event with "kill this Guardian Angel instead," and the unit is healed and recalled to your base.
• Death Prevented: Because the unit is recalled to the base, it never actually "dies" or is sent to the trash.
• Deathknell Requirement: According to Rule 808.1.d.1, the Deathknell trigger condition is specifically the permanent being killed and sent to the trash. If the "killed" event is replaced by a recall (as it is with Guardian Angel), the Deathknell will not occur.

This interaction is consistent with how other replacement effects, such as Zhonya's Hourglass, function in Riftbound.