Is the song My Heart Will Go On historical?

Not really - it is coincidental that it describes the (fictitious) affair between Rose and Jack.

An historical song would be something like Gordon Lightfoot's The Wreck of the Edmund Fitzgerald - about the American Great Lakes freighter that sank in 1975, or more obliquely Fotheringay by Fairport Convention about Mary Queen of Scots. But these don't really define social movements.

I woud go with something like The March of the Women by Ethel Smyth from 1911 - aka Shoulder to Shoulder. A protest song of the Suffragette movement in England.

More modern would be "19" by British musician Paul Hardcastle which was one of many anti Vietnam war songs - others (more generally ant-war) would be Eve of Destruction by Barry McGuire, or the European Green/CND song 99 Red Balloons by Nena.

Some of the 1960's protest movement songs would qualify. MHWGO definitely does not qualify because it does not discuss the event it only is one person's reaction to it. WYB does not either. You need something like "We Shall Overcome" as it is known to be the song of the civil rights movement.