-- Edit --
Operating Unity Version should read

Operating Unity Version : 2020.3.30f1

Step 1. of Repro should read

1. Install Unity Version 2020.3.30f1

Issue still ongoing.

Reported Resolution Version : 2020.1.0a24
Reported Errors :
GetLoadedImportedAssetsAndTargetHashes called with invalid targetHash
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
Assertion failed on expression: 'it->second.IsValid()'
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Operating Unity Version : 2020.3.0f1

Experienced Errors :
GetLoadedImportedAssetsAndArtifactIDs called with invalid artifactID
Assertion failed on expression: 'it->second.IsValid()'

Repro :

1. Install Unity version 2020.1.0f4
2. Install Visual Effects Graph from Package Manager
3. Remove Visual Effects Graph from 'In Projects' tab of Package Manager
4. Restart Unity and load project
5. Get Errors : GetLoadedImportedAssetsAndArtifactIDs called with invalid artifactID
Assertion failed on expression: 'it->second.IsValid()'

I can only assume this is still an atlas error for removed packages based on the error codes. That is why I posted here instead of opening a new complaint.

Happened to me just now in version 2019.3.6f1 after prefabbing an object.
Deleting the Library fold did not fix it, but now I get this warning multiple times until crash:

Identifier uniqueness violation: "'Name:minebot_front_upperleg, Type:Mesh'. Multiple Objects with the same name/type are generated by this Importer. There is no guarantee that subsequent imports of this asset will properly re-link to these targets.

This is definitely not fixed. Unity 2019.4.12f1 LTS. 68 error listed every time.

This is not fixed. Unity 2019.4.11f1 LTS. Appears every time the project is opened.

Getting each of these x4 on project opening in v2019.3.13f1 - can't recall when or why it started but it's definitely there once the scene loads.

Getting this in 2020.1.0b9

I have just downloaded the Unity Hub, installed latest version 2019.3.12f1 Personal and got this error just by importing a purchased asset from the store, whereas this issue suggests that this only happens on "deleting packed atlases"

Im getting this after just upgrading to 2019.3f11 and doing a build. It appears to be preventing the build from outputting. I am investigating further.

Will there be a fix for 2019.3?