@lucky Having checked the latest Unity 6 Preview 6000.0.0f1 (with all UPM packages updated), no such error occurred on our end. When jumping to the declaration of .spritesheet, we just see a normal Obsolete with no bool error=true, thus only being a warning:

//
// Summary:
//     Array representing the sections of the atlas corresponding to individual sprite
//     graphics.
[NativeProperty("SpriteMetaDatas")]
[Obsolete("Support for accessing sprite meta data through spritesheet has been removed. Please use the UnityEditor.U2D.Sprites.ISpriteEditorDataProvider interface instead.")]
public SpriteMetaData[] spritesheet { get; set; }

Could you please have a try whether updating resolves the issue for you?
Also I wonder where you got Unity version 2023.3.21f1 from. Could you share a download link, or how you installed this?

@lucky FYI: I have deleted the posting by Spinebot, which listed outdated 4.2-beta UPM package download links. Please never use old beta packages when non-beta 4.2 packages exist.

@lucky In general we always recommend setting up automated (scripted) exports of your skeleton assets, so that re-exporting all your assets and copying it to the target asset directory in a new version is no work at all (as Misaki mentioned above).

Since you explicitly asked: you can skip exporting your skeletons as version 4.1 and directly upgrade from 4.0 to 4.2. You probably get the best of both worlds by first upgrading your Unity project code-wise to spine-unity 4.1 (following the 4.0 to 4.1 upgrade guide) and copying this 4.1 project away for later testing. Then upgrade the project to 4.2 and paste all re-exported 4.2 skeleton assets there. If you now encounter any issues with a specific skeleton and don't know which upgrade broke it, you could still test if re-export of only the problematic skeleton as 4.1 works in your 4.1 Unity project or not.