Once you download the file (e.g., WhatsApp_2.18.200.xap ), verify its size. Legitimate final versions are roughly 45–55 MB. If it is 2 MB, it is malware or a stub.
Microsoft and WhatsApp had a close partnership in the early 2010s. The official download source was the (later rebranded Microsoft Store). whatsapp xap download link
Modding, data extraction, or operating system retro-fitting. Where to Safely Find Legacy XAP Files Once you download the file (e