To enable interpolation for Spine 2D animations in Unity, you need to ensure that your animations are being updated correctly in Unity. Here are some steps and tips to help you achieve smooth interpolation:

1. Playback View in Spine: In Spine, you can use the Playback view (Alt+P in animate mode) to change the playback speed and see how the animations look slower or faster. You can also turn off interpolation to see the difference. This helps you understand how your animations will look at different frame rates.

2. AnimationState.Update() in Unity: On the Unity side, the AnimationState.Update() method is called automatically by the SkeletonAnimation component, using Unity's Time.deltaTime as the frame delta. This ensures that your animations are updated smoothly based on Unity's frame rate.

3. Auto Reset and Additional Keys: If you are using SkeletonMecanim, you might want to enable Auto Reset and add additional keys at the first frame as described in the spine-unity documentation. This helps in maintaining the correct pose and smooth transitions between animations.

4. AnimatorOverrideController: If you are using SkeletonMecanim, you can use an AnimatorOverrideController to create one master controller and then override the used animations with the animations of another skeleton. This helps in managing multiple skeletons and their animations smoothly.

5. Disabling Interpolation in Script: If you want to disable interpolation in your script, you can refer to the example provided in the forum post here. This post shows how to change the refresh rate at runtime to mimic no interpolation.

6. Checking Animation in Preview: When you find something wrong with your animation, it is often helpful to check the appearance in the Preview of the Inspector window in Unity. If it looks wrong here, then something might be going wrong with the export from Spine to Unity.

By following these steps, you should be able to achieve smooth interpolation for your Spine 2D animations in Unity. If you still encounter issues, make sure to check the latest version of the spine-unity runtime and update your Spine editor if necessary.

Keyframe interpolation is always enabled, you would require special setup or code to disabled it.

Are you perhaps using single images (image sequences) at your attachments?
Or did you export your skeleton as Image or Video sequence instead of as Data (json or skel.bytes)?

acai I've found several posts mentioning holding shift or using code, but I couldn't quite understand them.

Holding shift? This makes no sense. In general it would help if you could share references to the postings that you don't understand, then we can explain what they were actually describing.

acai By referring to that thread, did you try fixing the FPS? In the thread you referred to, the poster initially speculated that the Interpolation setting might be having an effect, but it turned out that the problem was solved when it was discovered that the difference in the FPS at which Spine and Unity update animations was affecting the visual difference.
Even if you fixed the FPS in Unity, did you not get the results you were expecting? Or have you not tried it yet?