Plugin does not support the sidepanel-flyout from the core

[LukeTowers]

<?php $flyoutContent = Block::placeholder('sidepanel-flyout') ?>

                <div class="layout-row">
                    <div class="layout flyout-container"
                        <?php if ($flyoutContent): ?>
                            data-control="flyout"
                            data-flyout-width="400"
                            data-flyout-toggle="#layout-sidenav"
                        <?php endif ?>
                    >
                        <?php if ($flyoutContent): ?>
                            <div class="layout-cell flyout"> <?= $flyoutContent ?></div>
                        <?php endif ?>
                    </div>
                </div>

The above code is missing from layouts/default.php; as a result, the builder plugin does not function.