Adding feDropShadow to a vertical line in SVG makes it disappear

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You can’t use objectBoundingBox units if your shape has no height or width.

Keyword objectBoundingBox should not be used when the geometry of the applicable element has no width or no height, such as the case of a horizontal or vertical line, even when the line has actual thickness when viewed due to having a non-zero stroke width since stroke width is ignored for bounding box calculations. When the geometry of the applicable element has no width or height and objectBoundingBox is specified, then the given effect (e.g., a gradient or a filter) will be ignored.

The default for filterUnits is objectBoundingBox units so you need to change that to userSpaceOnUse i.e.

<svg preserveAspectRatio="xMinYMin meet" viewBox="0 0 21 484" xmlns="http://www.w3.org/2000/svg">
  <title>Line Drop Shadow</title>
  <description>A red line with 5px width thickness and round caps, having a drop-shadow. This highlights the regression documented in PURP-1017.</description>
  <defs>
    <filter id="dropShadow" filterUnits="userSpaceOnUse">
      <feDropShadow dx="4" dy="0" stdDeviation="4"></feDropShadow>
    </filter>
  </defs>
  <g id="Artboard" stroke-width="5" stroke="#FF0000" fill="#000000" stroke-linecap="round">
    <path style="filter: url(#dropShadow)" d="M7.5,8.5 L7.5,471.5" id="path-1"></path>
  </g>
</svg>

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