I forked the repo, modified entity-registry like so:

- name: transform
    category: core
    keyAspect: transformKey
    aspects:
      - transform

I created a Transform.pdl file (

datahub/metadata-models/src/main/pegasus/com/linkedin/transform/Transform.pdl

):

namespace com.linkedin.transform

@Aspect = {
  "name": "transform"
}
record Transform {
  name: string
}

And added a TransformKey (

datahub/metadata-models/src/main/pegasus/com/linkedin/metadata/key/TransformKey.pdl

):

namespace com.linkedin.metadata.key

/**
 * Key for a Transform
 */
@Aspect = {
  "name": "transformKey"
}
record TransformKey {

  /**
  * Unique id for the transform.
  */
  transformId: string
}

I rebuilt the Docker image, and executed this curl:

curl --location --request POST '<http://localhost:8080/aspects?action=ingestProposal>' \
--header 'X-RestLi-Protocol-Version: 2.0.0' \
--header 'Content-Type: application/json' \
--data-raw '{
  "proposal" : {
    "entityType": "transform",
    "entityUrn" : "urn:li:transform:123",
    "changeType" : "UPSERT",
    "aspectName" : "transform",
    "aspect" : {
      "value" : "{ \"name\": \"transform1\" }",
      "contentType": "application/json"
    }
  }
}'

If I change the entity type to a type (eg. transforma instead of transform), modify the aspect, or the urn, it bugs. But when I execute the curl as above, it works and returns

{"value":"urn:li:transform:123"}

Therefore, I assumed everything worked fine.

Nevertheless, if I go in the UI, I cannot find my entity. Is that because it wasn't ingested, or because it is simply not available in the search / UI ?

they are available via the API-s as you have confirmed

It's great to see such a vibrant community!