Refer to Product Change Notification ALAN-04ZEJC771 with an End of Life (EOL) notification published November 17th, 2021, for affected CPNs.
The EOL is related to AT89C51ID2 products in the 44-lead VQFN package referred to in the data sheet (ww1.microchip.com/downloads/en/DeviceDoc/doc4289.pdf). The reason for EOL is that there is no manufacturing support for this package.
Note that the T in the replacement CPNs AT89C51ID2T vs. the EOL CPNs AT89C51ID2 do not have any other significance than distinguishing replacement CPNs from EOL CPNs.
Refer to the AT89C51ID2 product page at www.microchip.com/en-us/product/AT89C51ID2 for a complete list of part numbers.