The phrase "has mostly remained" is grammatically correct and can be used in written English.
It is typically used to describe something that has continued to exist or persist in a certain state over time. Example: Despite several changes in leadership, the company's core values and principles have mostly remained unchanged over the past decade.
The carbon dioxide put underground has mostly remained there, preventing it from re-entering the atmosphere.
As a music journalist and historian, he has mostly remained outside academe, a rarity among American public intellectuals.
While the banner has been unfurled four or five times, it has mostly remained in the box in an attic for 17 years.
Delgado has mostly remained in his native Puerto Rico this off-season, recuperating from the broken left hand he sustained when hit by a pitch in the last game of the season, as the Mets completed their epic collapse.
Gilbert has been approached by outside investors in the past, but his original group has mostly remained intact since he bought the Cavaliers and the rights to operate their arena for $375 million from Gordon Gund in 2004.
Mr. Prabhakaran has mostly remained constant, a man who is ready "to take any method, however repulsive, as long as it furthered his struggle," according to a biography, "Inside an Elusive Mind: Prabhakaran," by M. R. Narayan Swamy.
Talks, they say, have mostly remained civil.
Unknowingly, he had mostly remained awake for 24-hour periods before sleeping.
The buildings, however, have mostly remained in some form of use.
Until last month, SAC had fully cooperated with the government's investigation, and its investors had mostly remained loyal.
And subscribers have mostly remained loyal, especially in Britain, where British Sky Broadcasting, the main pay-TV company, has more than 10 million customers.
