The docs for G_PARAM_DEPRECATED says:

 * @G_PARAM_DEPRECATED: the parameter is deprecated and will be removed
 *  in a future version. A warning will be generated if it is used
 *  while running with G_ENABLE_DIAGNOSTIC=1.

so the warning should be output only when that env var is set. That makes sense, since this warning is only ever useful to developers, and otherwise needlessly pollutes the journal or ~/.xsession-errors.

However, the code defaults to 'enable' if the env var is NOT set:

[gobject/gobject.c:]
static void
consider_issuing_property_deprecation_warning (const GParamSpec *pspec)
{
  static GHashTable *already_warned_table;
  static const gchar *enable_diagnostic;
  static GMutex already_warned_lock;
  gboolean already;

  if (!(pspec->flags & G_PARAM_DEPRECATED))
    return;

  if (g_once_init_enter (&enable_diagnostic))
    {
      const gchar *value = g_getenv ("G_ENABLE_DIAGNOSTIC");

      if (!value)
===>    value = "-";

      g_once_init_leave (&enable_diagnostic, value);
    }

  if (enable_diagnostic[0] == '0')
    return;


The fix is easy: change |value = "-";| with |value = "0";| above.