Always netstandard20 with Unity?

[Jenix-Park]

On this page, you said, "for master branch take DryWetMIDI.<release_number>-bin-netstandard20.zip archive..".
Is still the netstandard20 version recommended over net45 even with the latest Unity Editor?

[melanchall]

For latest Unity you should definitely use .NET Standard binary. Please note that there is DryWetMIDI asset on the Unity Asset Store to get rid of manual steps.

[Jenix-Park]

What happens if I use the net45 version?

Older Unity Editor definitely needed the API Compatibility level of .NET Standard 2.0, but starting the version 2018.1, the .NET 4.x has been supported as stated here.
So I put the DryWetMIDI-net45 files in some of my projects manually in the past and had no issues at all so far.

Do I need to switch all of them?
I don't use MIDI devices and playback features in those projects, by the way.

[melanchall]

Nothing bad will happen with .NET Framework version of the library. But .NET Standard is more modern technology than .NET Framework, so please use it. I believe .NET Framework support was added to cover more developers and to provide ability to use more old 3d party libraries written on .NET Framework.

More than that, from the linked article:

Unity recommends .NET Standard 2.0 for new projects.

So I recommend you to use .NET Standard.