Hello,

We would like to request a feature to restrict editing of custom fields to specific users and for those fields to be read-only for non-editing users.

Workaround:

While there is no current way to meet this requirement, some scenarios/teams might benefit from using Jira Cloud automation to some extent.
The idea is to use automation to track field changes. If a user from a specific user group (or not from a specific user group) triggers a field change, the automation rule will edit the field value to what it was prior to the edit.

The automation rule concept:

1) Trigger: Field value changed
Select the field to monitor for changes, for example the "Priority" field
2) Log Action:

Initial value - {{fieldChange.fromString}}, Modified value - {{fieldChange.toString}}

3) Add a User Condition
     User: User who triggered the event
     Check to perform: Is not in group / Is in group
     Criteria: Select a group to compare with
4) Log Action (Optional):

The user ({{initiator}}) who changed the field value is not part of the required group

5) Action: Edit Issue
Populate the "Additional fields" text box with a JSON payload like this:

{
  "fields": {
    "Priority": {"name": "{{fieldChange.fromString}}"}
  }
}

Note that not all fields might be supported and when working with custom fields, their ID must be referenced.
You can get all the fields from an issue by making a GET call like this:

https://{YourCloudInstance}.atlassian.net/rest/api/3/issue/{Issue-Key}?expand=names 

You could improve the rule, for example, to send an email to the unauthorized user (initiator.emailAddress) trying to make changes to the monitored field(s), etc.