If a unit or units have any persisting damage for example on the battlefield "Frozen Fortress" then I play Imperial Decree, would those units die?

No, units that already have damage marked on them from Frozen Fortress will not die simply because you play Imperial Decree.

According to RiftJudge FAQ #8633 and #9561, Imperial Decree only creates a triggered ability that kills units that take damage after the card has resolved and its effect has become active. It does not look back in time to check for damage that occurred before the card was played.

Here is the breakdown of why they do not die:

• Event vs. State: The trigger condition for Imperial Decree is the event of "taking damage." If a unit was already damaged by Frozen Fortress before you played Imperial Decree, that damage event has already occurred, and the condition is not met.
• No Retroactive Effect: Imperial Decree does not check the current state of damage on units; it only monitors for new damage events occurring while its effect is active.

Note: If any of those units were to take additional damage later in the same turn while Imperial Decree is active (for example, if you cast another spell that deals damage), the trigger would fire at that time, and those units would be killed.