Filters apply to: All elements. In SVG, it applies to container elements without the defs element, all graphics elements and the use element.
https://drafts.fxtf.org/filter-effects/#FilterProperty

Mix-blend-mode applies to: All elements. In SVG, it applies to container elements, graphics elements and graphics referencing elements.
https://drafts.fxtf.org/compositing-1/#mix-blend-mode

SVG container elements are: ‘a’, ‘clipPath’, ‘defs’, ‘g’, ‘marker’, ‘mask’, ‘pattern’, ‘svg’, ‘switch’, ‘symbol’ and ‘unknown’.
https://svgwg.org/svg2-draft/single-page.html#struct-TermContainerElement

Some of these container elements are not rendered (see: https://svgwg.org/svg2-draft/single-page.html#render-TermNeverRenderedElement). For example, <pattern>. I think it makes sense to restrict filter and mix-blend-mode to renderable elements (see: https://svgwg.org/svg2-draft/single-page.html#render-TermRenderableElement).

Firefox, Chromium, Edge, and Safari do not support these properties on <pattern>:
https://output.jsbin.com/gumusuq/2/quiet