Only the 2010 data is in the spatial data services. Given that Bing Maps is being retired, I doubt any new data sources will be added to that service.
That said, that boundary data is relatively small by today's standards and there really isn't a need to have this in the spatial data services as you can load that full set into JavaScript and add custom filtering to it (if needed) and it would perform well. The official boundaries are published here: https://www.census.gov/geographies/mapping-files/time-series/geo/cartographic-boundary.html
The highest resolution version of those boundaries is about a 6MB shapefile and which is good for analysis purposes but likely overkill for display purposes. Loading shapefiles into Bing Maps is a bit difficult, but relatively easy in Azure Maps. They have a medium resolution file (1:5M ratio) which is more than good enough for most display scenarios) and is less than 500KB in size. There is KML files for the other resolutions which would work in both Bing Maps and Azure Maps as-is, but are larger. Gzip, which would most likely happen automatically these days would compress the KML to about the same size as the shapefile. I personally prefer geojson files and have converted the high resolution and medium resolution files and made available here: https://1drv.ms/u/s!AgFqp4QkIjUNr9c6GBG0R7yC_aeZsA?e=w1dbJb When compressed, they are about the same size as the shapefile/kml files.
Using GeoJSON in both Bing Maps and Azure Maps is very easy, although Azure Maps is the easier of the two and much faster. Especially if you want to data driven styling (e.g. you want the polygon colors to change based on a data value from feed, like vote counts). Fill patterns are also supported in Azure Maps which can be useful when the vote counts are split/close (e.g. red/blue hashed lines).
If you want a quick test to see what the data looks like you can either download the shapefiles and drag/drop the zip file into this sample: https://samples.azuremaps.com/?search=shapefile&sample=drag-and-drop-shapefiles-onto-the-map
Or the GeoJSON files I provided and drag/drop one of them into here: https://samples.azuremaps.com/?search=drag&sample=drag-and-drop-geojson-file-onto-map
Here are some resources on using Azure Maps:
- Docs: https://zcusa.951200.xyz/en-us/azure/azure-maps/
- Around 300 examples: https://samples.azuremaps.com/