Trying to set a Name-Value type field to read-only fails using a UI Policy or a Client Script.
1. Create a new field of type Name-Value Pairs on the incident table.
2. Create a UI Policy to set the field created in step 1 to read-only.
3. Open any incident record and notice the field created in step 1 is not read-only as per policy.
After carefully considering the severity and frequency of this problem and the risk of attempting a fix, it has been decided to not address this issue in any current or future releases. We do not make these decisions lightly, and we apologize for any inconvenience.