<style>
input:disabled + span { color: gray; }
</style>
<p>
Player count:
<br>
<select name="players">
<option value="">4 Player</option>
<option value="3">3 Player</option>
<option value="2">2 Player</option>
</select>

<p>
<label>
<input type="checkbox" value="true" name="deluxe">Deluxe Edition Playtest
</label>

<p>
<label>
<input type="checkbox" value="true" name="emperor">Optional Emperor Rules
</label>

<p>
<label>
<input type="checkbox" value="true" name="no_demagogue" disabled><span>No Demagogue</span>
</label>

<script>
document.querySelector("select[name='scenario']").onchange = function (event) {
	let input = document.querySelector("input[name='no_demagogue']")
	if (/Expansion/.test(event.target.value)) {
		input.removeAttribute("disabled")
	} else {
		input.setAttribute("disabled", true)
		input.checked = false
	}
}
</script>