The phrase "longstanding debate" refers to a discussion or argument that has been happening for a long time, often without a clear resolution or consensus. Full definition
But retention has been the subject of longstanding debate. (gse.harvard.edu)
His findings, published by The Astrophysical Journal Letters, help to resolve longstanding debates about how these dwarf galaxies formed. (sciencedaily.com)
If Hoover's new analysis and interpretations hold up to scrutiny, the work could powerfully influence longstanding debates over the origins of terrestrial life and rarity of life elsewhere in the universe. (dotearth.blogs.nytimes.com)