Version 2.0

@matia.ward, I decided to add breadcrumb into Decoupled Kit Block (2.0.3 version).
Please, see system_breadcrumb_block block and settings/breadcrumb section in the resulted JSON.

Not sure if I'm doing something wrong, but it seems to give back the same breadcrumbs regardless of what the custom_path string is:

"settings": {
          "id": "system_breadcrumb_block",
          "label": "Breadcrumbs",
          "label_display": "0",
          "provider": "system",
          "breadcrumb": [
            {
              "text": "Home",
              "url": "/"
            },
            {
              "text": "Jsonapi",
              "url": "/jsonapi"
            }
          ]
        },

The breadcrumb output should change based on that variable, correct? Example URL string (whitelisted IPs only, so you won't be able to view the JSON): https://drupal.nidentistry.devsr.com/jsonapi/decoupled_kit/blocks?curren...

@matia.ward, ah, sorry, you are right. Core breadcrumb doesn't know how to construct breadcrumb correctly (it uses $this->context->getPathInfo() istead of $route_match data). Try to use Easy breadcrumb → or similar modules.

@matia.ward, I did commit to dev version, please check.

I updated a site to the 2.0 dev version and looked at the json response using this url: /jsonapi/decoupled_kit/blocks?current_path=/testimonials/single-post&current_theme=gin. My response now includes the parent page! The indexing is off though, goes from 0 then 2:

"drupal_internal__id": "gin_breadcrumbs",
        "theme": "gin",
        "region": "breadcrumb",
        "weight": 0,
        "provider": null,
        "plugin": "system_breadcrumb_block",
        "settings": {
          "id": "system_breadcrumb_block",
          "label": "Breadcrumbs",
          "label_display": "0",
          "provider": "system",
          "breadcrumb": {
            "0": {
              "text": "Home",
              "url": "/"
            },
            "2": {
              "text": "Testimonials",
              "url": "/testimonials"
            }
          }
        },

Tested with another nested child and got even more index weirdness, lol. This path was /testimonials/single-post/another-child.

"breadcrumb": {
            "0": {
              "text": "Home",
              "url": "/"
            },
            "2": {
              "text": "Single Post",
              "url": "/testimonials/single-post"
            },
            "3": {
              "text": "Testimonials",
              "url": "/testimonials"
            }
          }

I think just fixing up the indexing would do it! Thank you!

@matia.ward, many thanks for your testing.
I've fixed that case, please test it on the dev version.
But the right way is to use contributed breadrumb modules like Easy breadcrumb → .