Back to old-fashioned cricket.
Seymour Nurse passed away earlier this year: http://www.espncricinfo.com/westindies/content/player/52636.html
He was one of the reasonably strong members of the West Indies invincibles of the 1960s, playing 29 Tests between 1960 and 1969.
And he was one of the few to score a double century in his last Test, which was https://www.espncricinfo.com/series/17283/scorecard/63038/new-zealand-vs-west-indies-3rd-test-west-indies-tour-of-new-zealand-1968-69
These are the few who achieved this:
As we can see, Warner, Sandham and Ponsford scored more than Nurse’s 258 in their final Tests.
Spare a thought for Sandham. He was the first to score a Test 300, and his match total of 375 was a record until 1974 when Greg Chappell overtook it. But an average cricket fan of today (or, indeed, from the 1940s onward) probably would not have heard of him unless they were deep into history.
Now we see that Sandham scored 325 and 50 in his last Test: https://www.espncricinfo.com/series/17590/scorecard/62579/west-indies-vs-england-4th-test-england-tour-of-west-indies-1929-30
And Ponsford likewise scored 266 and 22 in his last Test (with Bradman making 244 and 77): https://www.espncricinfo.com/series/17565/scorecard/62623/england-vs-australia-5th-test-australia-tour-of-england-1934
It was not often that Bradman made a double century but was outscored by a team-mate.
So we see that SM Nurse, who made 258 in his only innings in his last Test, holds the record for the highest last innings score in all Tests.
Others to score a double century in their last Test innings are:
Aravinda de Silva; 206 in his last first-class match.
JN Gillespie; 201* as a night-watchman batting at No 3 instead of his usual 9.
Warner and Root will be around for a while.
For India, the answer to this is VM Merchant, with 154 vs England in 1951-52. VS Hazare outscored him with 164*. Other Indians with a century in their last Test innings are VL Manjrekar (102* vs NZ in 1964-65) and M Azharuddin (102 vs SA in 1999-2000).