<svg id="svg" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 1140 87" style="width: 1140px; height: 87px;">
<defs>
<filter id="shadow">
<feDropShadow dx="5p" dy="5" stdDeviation="3"/>
</filter>
</defs>
<path d="M 0 60 C 0 60 283 87 427 43.5 C 427 43.5 855 -60 1143 60 L 1143 0 0 0 Z"
style="stroke:red;fill:red;filter:url(#shadow);" />
</svg>
filter: drop-shadow - работает только с <image>