I see. I agree with you, but it would be pretty inefficient to fetch a single static value with AJAX. If you look at the source of the rendered panel page, you see:
<script>window.panel = {"url":"http://localhost/ikarov/panel","site":"http://localhost/ikarov","api":"http://localhost/ikarov/api","csrf":"9e1534bfa938023d64360efbfedb4ed29fa2bf7e1d41b71efc5d96d66c34d964","translation":"en","debug":true,"search":{"limit":10}}</script>
<script src="http://localhost/ikarov/media/panel/3d4b11f6ee2a89fd5ace87c910cee04b/js/plugins.js" defer></script>
<script src="http://localhost/ikarov/media/panel/3d4b11f6ee2a89fd5ace87c910cee04b/js/vendor.js" defer></script>
<script src="http://localhost/ikarov/media/plugins/index.js" defer></script>
<script src="http://localhost/ikarov/media/panel/3d4b11f6ee2a89fd5ace87c910cee04b/js/app.js" defer></script>
So the panel already uses this method to provide server data to the Vue app. Perhaps a feature could be implemented that allows you to add your own <script> tag. Or perhaps some key in the Kirby::plugin definition of the plugin where you can specify some values to be included in window.panel up top.