If you don't like that child forms from Lazarus are visible on the application bar is then do this.
1) If you use anchordocking then open components/anchordocking.pas and make this change:
constructor TAnchorDockHostSite.CreateNew(AOwner: TComponent; Num: Integer);
begin
//..
DragKind:=dkDock;
{$IfDef LINUX}ShowInTaskBar:=stNever;{$IfEnd} //<-- Add this !!
//...
end;
2) For not docked child forms open them and make same changes. I make here changes for ObjectInspector:
// components/ideintf/objectinspector.pp
constructor TObjectInspectorDlg.Create(AnOwner: TComponent);
//..
FComponentEditor := nil;
{$IfDef LINUX}ShowInTaskBar:=stNever; {$IfEnd} //<-- Add this !!
//..
end;