As a Jenkins user writing pipelines from my projects,
When I use a matrix in Declarative syntax because this syntax is really efficient and easy to read and maintain
Then I would want to allow defining dynamically some axis from a file content and/or a variable defined outside the scope of the pipeline block

Examples of the expectations (that are only suggestions):

• With a readYaml helper method:

pipeline {
  // ..
  stages {
    stage("Build") {
      matrix {
        axes {
          axis {
            name 'GOARCH'
            values readYaml file: 'archs.yml'
          }
// ...

with the following file:

# archs.yml
- amd64
- i368
- s390x

• Same idea with a JSON file (and the readJson helper):

//...
          axis {
            name 'GOARCH'
            values readJson file: 'archs.json'
          }
//...

["amd64", "i386","s390x"]

• Eventually with a dumb text file with 1 value per line:

//...
          axis {
            name 'GOARCH'
            values readTxt file: 'archs.txt
          }
//...

amd64
i386
s390x

• Or from a variable defined outside the `pipeline {}` block:

def architectureList = readYaml file: "archs.yaml"

pipeline {
//...
   axis { 
     name 'GOARCH' 
     values architectureList
  }

Of course, the scripted syntax solves this issue, but the cost of switching syntax is high (not mentioning that it does not fit all the pipeline writers).

Inspiration come from https://github.blog/changelog/2020-04-15-github-actions-new-workflow-features/#new-fromjson-method-in-expressions.

It's interesting how CDS and Gitlab are "solving" the dynamic matrix issue by using "templates" that generate sub pipelines for each matrix axe: https://ovh.github.io/cds/docs/concepts/template/ and https://gitlab.com/gitlab-com/www-gitlab-com/-/merge_requests/45795/diffs.