Taxonomy search return nested results from sub level terms if configured

Taxonomy search result show only the direct contents of the a term. In many cases we have parent/ancestor hierarchical taxonomy terms. If we select a parent term in the custom search form, we may also want to get results from all children taxonomy terms.

I know it's possible to do it by manually selecting all sub terms in the multiple selection drop-down list, but if you have complex taxonomy hierarchy with hundreds of terms, this will be extremely painful to select from a top level term.

I would really like to have a configurable checkbox in the custom search block config form, if checked, taxonomy search will return all nested results rather than just the direct results.

I found similar issues for old versions of Drupal,

for D7
https://www.drupal.org/node/2342301
for D6:
https://www.drupal.org/node/1165538

I don't known if they are fixed or not, but in 8.x-1.x, this function is still missing.