Also, this will prevent you from editing any of these, since as soon as you edit it it will immediately switch to "full" I think.

Hmm, true, you can't rename a Dashboard, but I don't think there is anything specifically interesting there otherwise?

Yeah, just surprising if you hit edit, don't change anything, hit save, and accidentally completely broke the dashboard in a way you can't revert.

You could make a change in PhabricatorDashboardEditController instead, more like this (near line 132):

$layout_mode_options = PhabricatorDashboardLayoutConfig::getLayoutModeSelectOptions();

$legacy = array(
  PhabricatorDashboardLayoutConfig::MODE_HALF_AND_HALF,
  PhabricatorDashboardLayoutConfig::MODE_THIRD_AND_THIRDS,
);

foreach ($legacy as $legacy_option) {
  if ($v_layout_mode == $legacy_option) {
    // Don't remove it if the dashboard is currently set to this value.
    continue;
  }
  unset($layout_mode_options[$legacy_option]);
}

Then each option will appear if the dashboard is already set to that value, but not for new dashboards.