Reproduction steps:
1. Open the attached “BugRepro” project
2. Open the “Assets/AddressableAssetsData/AddressableAssetSettings.asset“ asset
3. Check “Enable JSON Catalog” in the Inspector window
4. Observe the Console window

Expected result: No errors are thrown
Actual result: “Library\PackageCache\com.unity.addressables@2581a6be7421\Runtime\ResourceLocators\ContentCatalogData.cs(486,10): error CS0246: The type or namespace name 'FormerlySerializedAsAttribute' could not be found (are you missing a using directive or an assembly reference?)” errors are thrown

Reproducible with: 2.5.0 (6000.0.51f1, 6000.1.7f1, 6000.2.0b4)
Not reproducible with: 2.4.6 (6000.0.51f1, 6000.1.7f1, 6000.2.0b4)
Fixed in: 2.6.0 (6000.0.51f1, 6000.1.7f1, 6000.2.0b4)

Reproducible on: Windows 11
Not reproducible on: No other environment tested