Simply because all arcgis js v3.x api will retired July 1, 2024.
https://support.esri.com/en-us/products/arcgis-api-for-javascript/life-cycle
V3.x and V4.x is completely different things. You have to disgard your v3.x apps. Re-create from scratch using v4.x api.
Why ESRI does that? Does not make sense at all. But you have no choice.
V4.x version table
https://support.esri.com/en-us/products/arcgis-maps-sdk-for-javascript/life-cycle
TransparentGov.net is always use lastest arcgis javascript api , up to date is V4.29