"undiminished interest" is a correct and usable phrase in written English.
You can use it to refer to an interest that is still strong and does not seem to be decreasing. Example: Despite the passage of time, the professor's undiminished interest in the language was clear.
The beautiful wild horses of the Camargue nature reserve (France), the wild ponies of the New Forest (England), and the Barbary macaques (Gibraltar) continue undiminished in popular interest.
He, by contrast, advocated "counter-cyclical policies"—lower interest rates and undiminished public spending, which might offset a collapse in private demand.The fund had meekly absorbed round after round of punishment from Mr Stiglitz.
Thanks to the slightly more relaxed control exercised over Bonner, she was able to maintain a degree of contact with the outside world, as well as to keep Sakharov's name in the world's press, whose interest in them was undiminished.
People's interest in politics is "undiminished", the survey said, but a trend towards "greater voter disconnection" had continued in this Parliament, with 53% of people saying the government does not much care "what people like me think".
In this way only the display is restricted to nodes of interest, but the accuracy undiminished since the data from all organisms is used every time; the tree topology is not recalculated using only the reduced subset of organisms.
Although greatly obstructed by Ranavalona's policies, French and British political interests in Madagascar remained undiminished.
Mr. Muldoon's interest in difficult poetics is undiminished in "Maggot".
He said the paper's "commitment to national news of interest to our readers is undiminished".
Despite his view that things in general are going downhill, Barzun is undiminished in his scholarship, research and polymathic interests.
"Despite the financial crisis, people's interest in diving, climbing and walking remains undiminished.
