When the Elimination match ends, you can still capture the flag, and when the scoreboard and the awards/crates are shown, you get the points for the flag.
I am not sure if the points count in the victory screen, but if you have a challenge that says you have to take the flag, that still counts..
Vigor ID: e8b3dca7-c233-4918-86a1-d84bb1be34bc
Video Evidence: https://youtu.be/cK-jnTm1c6Y
Step 1. In the last round of the Elimination game, wait till the flag spawns in.
Step 2. Kill the last player alive on the enemy team.
Step 3. Take the flag, right before the black screen comes.
Step 4. Recive your 250 points, in the victory screen.
I am not sure if the points count in the victory screen, but if you have a challenge that says you have to take the flag, that still counts..