The phrase "a constant subject of" is correct and usable in written English.
It can be used to describe something that is frequently discussed or a recurring topic of interest.
Example: "The impact of climate change has become a constant subject of debate among scientists and policymakers."
Alternatives: "a frequent topic of" or "a regular theme of".
Around the market, police and immigration raids have become a constant subject of discussion.
This issue has been a constant subject of gossip and conjecture at Westminster, around Whitehall and in the media.
One might have expected that a man with his sympathies would have been a constant subject of reproach and complaint.
In this city, as in most others in France these days, crime and how to combat it is a constant subject of discussion.
"The increased awareness around the long-term impact of head trauma on men that played football has been a constant subject of conversation among the players".
Democrats would partially overlap with conservatives or Christian Democrats here, but the absence of any sort of electable socialist movement in America is a constant subject of incomprehension.
Donovan, a native Californian, is the team's top draw, but his rights are owned by Bayer Leverkusen of Germany and he is the constant subject of transfer rumors.
In the mid-1970s he moved south with his wife Marie - the constant subject of Heaney's passionate, tender and rueful love poems - and their children, first to a cottage in County Wicklow, and later to Dublin.
Even less does anyone get credit for realizing that the human body was a natural subject for this new invention, given that the body had been the most constant subject of art since the beginning of human time.
The housing shortage in Brazil, which is currently estimated at 5.5 million units, is the constant subject of architectural and urbanistic studies.
