Best workaround I've found is to make non public fields protected. It gets rid of the warning and has the added benefit of making it impossible to name mangle serialized fields across subclasses.

People having this issue:
You can also set the field as public, this way also aligns more with the view of "Component Oriented Programming" as components are not designed to be operated alone, and are not expected to be "self contained"

There is nothing wrong in using public, and it will help you find other patterns that are neater.

Purists may think this is wrong. But just think in the project and the best way to make your life easier working on it. You don't have to live to c# compiler developers expectations, you just need to make your game, and make it run good and fast. It is much simpler if you don't have to wonder "should this thing be serializefield private, or public instead".

I invite you to leave that dogmatic OOP "things shouldn't be public because they're not meant to be edited from outside". Most of the time, you are waisting brain time thinking on that. And when it really matters you will have clear motivations about why you need to do one think or another.

In the meantime, don't bother and use public.

People who are (rightfully) shocked that this is not fixed => PLEASE VOTE FOR THIS BUG.

Commenting here is not going to help but voting will.
(If every commenter had voted, it would be long fixed.)

You could simply assign null to the serialized field instead of using a hacky workaround like #pragma warning disable 0649.

E.g.:
[SerializedField]
private GameObject test = null;

Argh!! It appears csc.rsp and mcs.rsp files are no longer being read in 2020.1.0b8? Is this another bug with recent changes to Visual Studio Projects?

* Other VS integration bugs include it randomly deciding to detach from VS such that the default .cs file viewer is used instead of VS when opening scripts from Unity.
* The VS debugger refuses to start until VS is restarted.
* And the super annoying "Your project has changed, do you wish to reload?" is back.

I had none of these problems in 2019.3 until upgrading to 2020.1. Now I end up restarting VS about 30 times a day.

six years, seriousely?
This really needs to be fixxed.

reported just right before found out we have open issue, anyway let it be 2

I understand that it seems to you that you better know what we need, but we really need it, so be so kind as to fix it as soon as possible.

oops! wrong ticket

this has been "postponed" for 6 years. Can we get a fix please?