The phrase "are a key subject" is correct and usable in written English.
It can be used when discussing important topics or themes in a particular context, such as academic writing or analysis.
Example: "In the study of climate change, renewable energy sources are a key subject that requires further exploration."
Alternatives: "are a crucial topic" or "are an important issue".
Down-hole operating condition diagnosis based on dynamometer card is a key subject for sucker rod pumping in oil extraction engineering.
Journeys on foot were a key subject in 19th-century European art, and are celebrated here in 100 artworks by Caspar David Friedrich, Norway's Johan Christian Dahl, Switzerland's Ferdinand Hodler, Paul Gauguin, Pierre-Auguste Renoir and others.
I've known Dr. Kengo Kuma of the University of Tokyo, an internationally respected architect, for a long time; he was a key subject in my second book, and he has a large, international staff.
Ashley Walters, actor and former rapper with So Solid Crew, is keen to talk about fatherhood, not only because he is in the market for some new wheels – a pram, to be precise – but also because modern fatherhood will be a key subject when he appears, later this month, at the Being a Man festival at the Southbank Centre in London.
Due to the long lasting consequences, school-to-work related inequality of educational opportunity (IEO) has been a key subject of empirical educational research for years.
The issue is likely to be a key subject of debate when the Conservatives meet for their annual conference in Birmingham on Sunday.
"The state of F1 has been a key subject within the sport for several months, and this weekend F1 commercial boss Bernie Ecclestone has given an interview on the subject to BBC F1 chief analyst Eddie Jordan.
Reducing carbon emissions from the transport sector is essential to meet the Paris Agreement emission reduction pledges made in 2015, and is a key subject of debate at the two-week U.N. climate conference currently meeting in Poland.
If the prediction sounds familiar it's because de Grey – this time described as a gerontologist – was subject to a similar profile in Friday's New York Times, thanks to his being a key subject of Jonathan Weiner's book Long For This World: The Strange Science of Immortality.
