UnityFoldoutGroupAttribute.cs

using Sirenix.OdinInspector;
using UnityEngine;

#if UNITY_EDITOR
using Sirenix.OdinInspector.Editor;
using Sirenix.Utilities.Editor;
#endif

public class UnityFoldoutGroupAttribute : PropertyGroupAttribute
{
    public bool BoldLabel;

    public UnityFoldoutGroupAttribute(string groupId, bool boldLabel = false) 
        : base(groupId) => BoldLabel = boldLabel;

    public UnityFoldoutGroupAttribute(string groupId, float order, bool boldLabel) 
        : base(groupId, order) => BoldLabel = boldLabel;
}

#if UNITY_EDITOR
public class PropertySelectorGroupAttributeDrawer 
    : OdinGroupDrawer<UnityFoldoutGroupAttribute>
{
    private GUIStyle foldoutStyle;

    protected override void Initialize()
    {
        foldoutStyle = new GUIStyle("foldout")
        {
            fontStyle = Attribute.BoldLabel ? FontStyle.Bold : FontStyle.Normal
        };
    }

	protected override void DrawPropertyLayout(GUIContent label)
	{
        Property.State.Expanded = SirenixEditorGUI.Foldout(
            Property.State.Expanded, label, foldoutStyle);

        if (Property.State.Expanded)
        {
            foreach (var child in Property.Children)
            {
                child.Draw(child.Label);
            }
        }
	}
}
#endif