Social Ratio Auto resize

It's quite impossible to have a script do this automatically because different people set up their layers and keyframes differently and if expressions are involved then this makes it even more awkward. 

However, I do have a system - https://youtu.be/HHn2pwWqPWA
and it's quite robust. So, you can build your own ADAPTIVE TOOL/SYSTEM    if you can and others have - not just me.
I also teach this in my advanced AE Courses. I actually think I can automate a lot of the processes but there are a few gaps and I've not had any urgency to work further with it then with what I have. 

On to your Q, it's a lot like Web development - you have options for Full Code, Lo-Code and No-Code. And in terms of Adaptive and even Responsive features, these too are the options you have. 

The automated way I have come up with is to create a MOGRT in a 4K comp for all of the graphics and animations and then add a simple Menu Control.

I have created about forty of these MOGRTs using single Expression Controls/Menu Control with menu items for HD, Social Square, Social Vertical, and 4K options in the Menu. A simple if/else expression for the Camera Zoom, scale, position, and sometimes rotation properties of all graphics and layers that need to be adjusted for the different sizes is added. The master MOGRT is in a 4K comp. The Essential Graphics workspace makes this a fairly simple task and simplifies the workflow. 

Here's an expression that changes the Camera Zoom in a complex comp to properly frame up all elements for the different frame sizes. I use zoom because perspective is controlled by camera position, and zoom (focal length) controls the framing. 

ctrl = thisComp.layer("Controller").effect("Comp Size")("Menu");
if (ctrl == 1){
	value / 2;
}
else if (ctrl == 2){
	value / 2.5;
}
else if (ctrl == 3){
	value
}

A simple expression to the Point of Interest for the camera to adjust the baseline of the graphics. Another similar expression tied to the Menu Control moves the position of a background layer, and the point of interest for some spotlights optimizes the lighting. A single-shape layer set as a guide layer with a couple of stroked rectangles gives me a good visual queue to set up the animations for each frame size. 

The video editing is done in Premiere Pro using the timeline templates for all social media workflows. The editor then adds the graphic to each of the timelines and selects the appropriate menu item. My clients love the workflow, and they can easily create different projects for different social media platforms while maintaining their branding standards in just a few minutes. 

Only one 4K comp is needed to set everything up, finalize the main comp, and export the MOGRT for all the projects you may need to produce. Premiere Pro makes editing the background content easier and faster than doing everything in After Effects.

If you are stuck with After Effects, a main 4K graphics comp with this menu control can be nested in your additional social media comps. The renders are then sent to the Media Encoder (most efficient for social media H.264 renders). 

Thanks for writing in. I am not aware of any specific feature that could resize your project in After Effects. However, Premiere Pro does have that feature: https://adobe.ly/499kNcN

If your video doesn't have embedded text, this feature could work pretty well. Hope it helps.