We use SetHasAnimations(true) and load atlas PNGs into the plugin's animation frames at import time (editor-side). At runtime we need to enumerate those frames (legacy equivalent: GetObjectClass().GetAnimations()[0].GetFrames()), but we can't find a documented SDK v2 runtime API for this.
I don't believe animated plugins were ever officially documented and supported even in SDKv1. It's something I am very reluctant to expose, because animated plugins have a potentially very large API surface, and really it all only exists specifically for the Sprite object at the moment. We cannot implement it just for one plugin specifically, since if it's documented and accessible, then other plugins will start to depend on it, and then we end up dealing with all the potentially painful and limiting long-term maintenance implications of having external addons depending on Construct's animation subsystem.
I can't recall the full history but I think it might be that Construct 3 only supported this to the same extent Construct 2 did solely for the benefit of the Spriter addon, as I think that is the only addon ever published that used the undocumented animation APIs. The intent of SDKv2 was to eliminate the possibility of accessing undocumented APIs precisely to avoid ending up with these kinds of accidental compatibility problems. I don't think there are any easy options here - to me the best thing to do is remove the use of animations from the Spriter plugin if you can, and find some other way to do what you need.
• Set origin/hotspot (SetOriginX/setOriginX, SetOriginY/setOriginY)
• Enable/disable collision (SetCollisionEnabled/setCollisionEnabled)
All these are available on IWorldInstance.
• Instance UID (GetUID(), getUID(), getUid(), .uid)
• Container siblings (GetSiblings(), getSiblings())
Available on IInstance. (Note the methods have changed to refer to "other container instances" rather than the term "siblings".)
• Invalidate bbox (SetBboxChanged/setBboxChanged)
This API is no longer needed with SDKv2.
• Enumerate instances (instances(), getAllInstances(), GetInstances())
• Object type name (GetName(), .name)
• Picked/paired instance (getPickedInstances(), getFirstPickedInstance(), getPairedInstance())
All available on IObjectClass.
• Sprite frame index (read) (_currentFrameIndex, currentFrameIndex, animationFrame)
• Sprite frame index (write) (_DoChangeAnimFrame(), _changeAnimFrameIndex, animationFrame)
Available on ISpriteInstance via animationFrame property
• No-premultiply blend (renderer.SetNoPremultiplyAlphaBlend/setNoPremultiplyAlphaBlend)
Not sure why you need this as Construct always uses premultiplied alpha blending, which is necessary for correct rendering. These methods have actually been completely removed from the underlying engine as nothing ever needed them.