The differentiation of .NET Core allowed the team to leave legacy problems behind and focus on multi-platform first and foremost. Now that .NET Core has been established and has more or less reached parity with .NET Framework, there's little reason to continue promoting the latter, and thus the name "Core" is redundant because it is the only future for the .NET platform.

I think it's safe to think of this not as abandoning .NET Core but conversely they're promoting the technology behind .NET Core as THE future.

I would expect the migration from .NET Core 3.1 to .NET 5 to be quite straight forward.