to cross the line = to change from being acceptable to being unacceptable; to do something wrong
Cambridge Dictionary of American Idioms 2006

This is closest in wording to the Russian idiom
"уходить/уйти за флажки" = break [an] unwritten rule[s], law[s]
which was inspired by the song by Vladimir Vysotsky about wolf hunting, where the narrator is a wolf

Explanation of the idiom:
In a traditional Russian wolf-hunting method, a wolf pack is encircled with a long rope having small swatches of fabric (флажки) stitched to it every few feet
Since it retains a human scent for several days, wolves tend to stay within the encircled area
In effect, the rope is a line the wolves cannot cross
In Vysotsky's song:
"For a wolf cannot break with tradition.
With milk sucked from the she-wolf's dugs
The blind cubs learn the stern prohibition
Never, never to cross the red flags!"
In the end, the wolf is crossing the line both physically and figuratively [ibid.]:
"But revolt and the life-force are stronger
Than the fear that the red flags instil
From behind come dismayed cries of anger
As I cheat them, with joy, of their kill"
(c) Kathryn Hamilton
http://www.wysotsky.com/1033.htm

The meaning is somewhat similar to
to go too far = to do more than is acceptable
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs 2002

However, whereas it is easy to imagine someone "going too far" accidentally, "crossing the line" has a stronger connotation of unwillingness to go by the rules
This is exactly the meaning of the context:
To the author, Kashin's words express an unwillingness to follow rules that have shifted over a period of five years

Thus, the entire sentence can be translated this way:
After I read the interview, I had a feeling it was [about] "crossing the line"