<select name="my_variable">
<option value="hidden" class="d-none">I want to momentarily hide this</option>
<option value="not_hidden">I just to see this one right now</option>
</select>
<select name="my_variable">
<option value="">Please select...</option>
<option value="hidden" hidden>I want to momentarily hide this</option>
<option value="not_hidden">I just to see this one right now</option>
</select>
Add an empty (invalid) option at the beginnning as a placeholder!
Of course; depending on the javascript library/framework (or none) you will mostly like have the functionality available once you can target the element, e.g.
let option = document.querySelector('option[value=hidden]');
option.setAttribute("disabled", "");