Custom Formatter for Read-Only Fields conditional based on other columns

The fieldsettings readonly property is not expression-capable. In Microsoft’s current list-form schema it accepts a Boolean value, so the formula string is not evaluated; it cannot switch editability by the current viewer or a multi-person column. Form formatting also changes only presentation, not the item’s permissions, so it must not be used as an authorization control. If this is only a user-interface requirement, customize the form with Power Apps and set the Comments card’s DisplayMode from membership logic. If users must be technically prevented from changing Comments, enforce that separately through SharePoint permissions or redesign the process: restrict item editing, store protected comments in a separately permissioned list, or have a controlled service write the field. Test with a member, not just an owner. Standard JSON can make Comments always read-only with a literal true, but conditional read-only behavior based on the viewer is not supported by fieldsettings