Seeing this in CI as well:
D:\a\spine-runtimes\spine-runtimes\spine-godot\spine_godot\SpineSprite.cpp(183): error C2664: 'void VisualServer::canvas_item_add_triangle_array(RID,const Vector<int> &,const Vector<Vector2> &,const Vector<Color> &,const Vector<Vector2> &,const Vector<int> &,const Vector<float> &,RID,int,RID,bool,bool)': cannot convert argument 11 from 'RID' to 'bool'
D:\a\spine-runtimes\spine-runtimes\spine-godot\spine_godot\SpineSprite.cpp(193): note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
D:\a\spine-runtimes\spine-runtimes\spine-godot\godot\servers/visual_server.h(1047): note: see declaration of 'VisualServer::canvas_item_add_triangle_array'
D:\a\spine-runtimes\spine-runtimes\spine-godot\spine_godot\SpineSprite.cpp(183): note: while trying to match the argument list '(RID, const Vector<int>, const Vector<Vector2>, const Vector<Color>, const Vector<Vector2>, Vector<int>, Vector<float>, RID, int, RID, RID)'
D:\a\spine-runtimes\spine-runtimes\spine-godot\spine_godot\SpineSprite.cpp(402): error C2660: 'VisualServer::canvas_item_add_mesh': function does not take 7 arguments
D:\a\spine-runtimes\spine-runtimes\spine-godot\godot\servers/visual_server.h(1048): note: see declaration of 'VisualServer::canvas_item_add_mesh'
D:\a\spine-runtimes\spine-runtimes\spine-godot\spine_godot\SpineSprite.cpp(402): note: while trying to match the argument list '(RID, RID, Transform2D, Color, RID, RID, RID)'
It looks like Godot 3.6 changed the signature of these functions in visual server.
I've opened an issue here:
EsotericSoftware/spine-runtimes2925