I have a NuGet feed for another source of packages for my project that I'd like to add to the Azure Dev Ops artifacts section as an upstream source, joining the standard NuGet feed. However, despite what the documentation says (https://learn.microsoft.com/en-us/azure/devops/artifacts/how-to/set-up-upstream-sources?view=azure-devops), the only custom option I see is for "custom NPN registry" with a note saying npm is the only package type that supports custom public upstream sources.
I added my feed anyway but it's not findable in Visual Studio's package manager, though I was able to add something from the standard NuGet feed. Is there a setting I'm missing to enable custom NuGet feeds to be added in, or are the docs incorrect here?
I believe you are misreading the documentation.
Custom public upstream sources are only supported with npm registries.
If you want to use a single feed for everything, you'll have to implement a bit of a hacky workaround.
The workaround, as unfortunate as it may be, would be to use a scheduled pipeline to synchronize the external feed with your Azure DevOps feed.