Why do you create backlog tasks or sprint tasks instead of using Columns on Workboards or Milestones in Projects?

We generally expect Agile/Scrum/Kanban-style workflows to use Columns to organize work stages and Milestones to organize sprints/iterations, so I'd like to understand why these tools are falling short in your workflow.

Well, we do use Workboards and Milestones, Subprojects, but we'd like to have that feature. We have multiple custom fields configured, which are relevant to different parts of the team. I would like to be able to configure different sets of forms for those subteams so our newcomers don't break things they don't yet understand or shouldn't touch.

As a simple example: let there be a somewhat big team of people, out of which there are developers, QAs, designers, analysts, managers and customer representatives. I'd expect customer to not have write access to any field but task description and title, even if they created that task; developers, designers to not have write access to fields used by QAs only (eg. links to external test-case management systems), etc.

We don't have exactly that situation, our main problems are newcommers changing things that they shouldn't and general clutter.

You should already be able to use custom forms to limit access to advanced fields by user, as we do on this install. If you (a user who is not a member of Community) try to create a new subtask here, I believe you will find that you can not access fields like "Assigned To" or "Priority". However, members of Community can.

I think the proposed feature (giving users a choice between different creation forms when they "Create Subtask") wouldn't help with the problem you describe (limiting access to advanced fields to advanced users). If we implemented the feature described here, novice users would be given a choice between "Create Task (Advanced)" and "Create Task (Simple)" and could still just click "Create Task (Advanced)".

Do you and @mirrom work together? Or are you perhaps describing two completely different problems faced by two completely different installs?

No, I do not work together with @dtf, but I guess we have a similar workflow.

As described in T12277 we also use milestone projects as sprint projects and make use of the several workboard columns. For us every sprint task is connected to a product backlog task, so the easiest way to create a new sprint task is by adding it from the product backlog task by clicking on Create Subtask. This automatically takes all project tags, priority etc. from the parent product backlog task. All our sprint task have a typical description:

At the moment our workflow is to create a subtask and then manually copy the default description into the new task. It would help us to use a predefined form as subtask so that this description (and some custom fields we use) are filled automatically.

An example.

There are "Create Task", "Create Bug", and "Advanced Create" forms. Basic forms are marked as "create" forms, advanced one is both "create" and "edit". I have access to all three of them.
When using a workflow similar to yours with "umbrella tasks", I'd like to be able to select which form to use when I click "Create Subtask".