After the 2018 errata, the disintegrate spell description now reads:

A creature targeted by this spell must make a Dexterity saving throw. On a failed save, the target takes 10d6 + 40 force damage. The target is disintegrated if this damage leaves it with 0 hit points.

If you were to polymorph an enemy into a rat and then disintegrate it, would the enemy be disintegrated or would it just return to its original form? I know that for Druids, it’s not an instant kill anymore, but is this the case for polymorph as well?

No, it wouldn't be disintegrated, just reverted (assuming the remaining damage doesn't reduce its original form to 0 HP)

The polymorph spell description states:

The transformation lasts for the duration, or until the target drops to 0 hit points or dies. [...]

The target assumes the hit points of its new form. When it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form. As long as the excess damage doesn't reduce the creature's normal form to 0 hit points, it isn't knocked unconscious.

So, as before, the polymorph spell ends and the polymorphed creature is reverted to its original form if its polymorphed form is reduced to 0 HP.

As you've noted, the description of the disintegrate spell has changed. Originally, it read:

On a failed save, the target takes 10d6 + 40 force damage. If this damage reduces the target to 0 hit points, it is disintegrated.

After the errata, it now reads:

On a failed save, the target takes 10d6 + 40 force damage. The target is disintegrated if this damage leaves it with 0 hit points.

The original wording caused 2 strange issues: the spell would not disintegrate creatures that were already at 0 HP before being hit by the spell, and the spell would disintegrate polymorphed creatures or druids in Wild Shape whose new form was reduced to 0 HP even momentarily by the spell (even if their original form could withstand the leftover damage).

The new wording after the errata fixes both issues. In this case, a target that fails the Dexterity saving throw still takes 10d6 + 40 force damage - but if it reduces their new form to 0, it doesn't instantly disintegrate them. They are simply reverted, and the leftover damage is applied to their original form. If their original form's HP is reduced to 0, only then would such a target be disintegrated.