.NET 9 will absolutely load a library that was built targeting .NET 5.

For instance, GraphQL was built targeting both .NET 5 and .NET Standard 2.0, as you can see towards the top of the NuGet page: https://www.nuget.org/packages/GraphQL

.NET 9 will use the .NET 5 build, not .NET Standard 2.0. (.NET Framework 4.8.x would use .NET Standard 2.0.) Because .NET 5 > .NET Standard 2.0.

Or Automapper 14 targets only .NET 8: https://www.nuget.org/packages/AutoMapper

It runs on .NET 9 and .NET 10.

Just to pick two examples at mostly random from the top packages on NuGet.