In 721 BCE.....that's "BCE" (before the common era). Or in other words "BEFORE CHRIST".
The Assyrian army captured the Israelite Capital at Samaria and carried away the people of the NORTHERN KINGDOM OF ISRAEL
into captivity.
Over 700 years BEFORE CHRIST.....so please, you can do better than that .
Try reading the scriptures and you will see that before the Assyrian captivity many of the northern tribes of Israel had migrated to Jerusalem and Judah.
During the reign of Rehoboam of the southern kingdom and Jeroboam of the northern kingdom when he (Jeroboam ) set up idol worship and rejected the Levite’s as priests, many of the northern tribes left for Jerusalem.
( 2 Chronicles 11:1-16)
During the reign of Asa of the southern kingdom more from the northern tribes of Israel came to Jerusalem. ( 2 Chronicles 15:9)
A decade after the Assyrian captivity, during the reign of King Hezekiah of Judah more from the northern tribes came to Jerusalem. ( 2 Chronicles 30: 1-27)
The Assyrian deportation never removed ALL the people. Many from all the northern tribes remained in the land. ( 2 Chronicles 30:6)
Even before the Assyrian invasion all the tribes were represented in the southern kingdom. God addressed them, speaking through Rehoboam, the son of Solomon. ( 2 Chronicles 11:3)
Approximately eighty years after the Assyrian captivity King Josiah issued a call and received offerings to repair the house of the Lord from Manasseh, Ephraim and all the remnant of Israel, all Judah and Benjamin. ( 2 Chronicles 34:9)
Josiah kept the Passover and all the Levite’s. all Judah and ALL Israel were present in Jerusalem.
( 2 Chronicles 35:17-18)
The point is that the northern tribes of Israel were never fully removed from the land, many had relocated to Judah and Jerusalem. And if a similar situation occurred as did after the Babylonian captivity, many people likely returned to the land of Israel.
They were never lost. The lost ten tribes idea is false.