And if people enjoy that kind of thing, they should read it!
I don't much. Many of the historical romances I read use modern-ish language and avoid some of the more unpalatable attitudes of the past. Yet I accept many of them as true-to-the time period.
And I asked myself, How does a contemporarily-written historical romance avoid sounding too modern?
The solution, I determined, comes down to the characters' mindsets.
In one of the many Jane Eyres I've watched, Jane Eyre and Helen are introduced as two giggling girls with beautiful hair. They bond when they both have their hair cut off (the event is borrowed from a minor character in the book). The haircutting is preceded with a show of bravado as they both coyly offer up their hair to the bad headmaster. It's the equivalent of the obnoxious junior high girls who say, "Oh, absolutely, whatever you say" to the teacher in fake compliant voices.
Lowood is a horrible place, and the headmaster is a horrible guy. But the confident, quirky, sarcastic behavior of the girls does not fit the time period.
Consider that Jane is inwardly passionate and defiant and Helen is dying. What do they actually say to each other in Bronte's novel?
[Jane said,] “Helen, why do you stay with a girl whom everybody believes to be a liar?”
“Everybody, Jane? Why, there are only eighty people who have heard you called so, and the world contains hundreds of millions.”
“How can they pity me after what Mr. Brocklehurst has said?”
“Mr. Brocklehurst is not a god: nor is he even a great and admired man: he is little liked here; he never took steps to make himself liked. Had he treated you as an especial favourite, you would have found enemies...Besides, Jane”—she paused. “If all the world hated you, and believed you wicked, while your own conscience approved you, and absolved you from guilt, you would not be without friends.”
“No; I know I should think well of myself; but that is not enough: if others don’t love me I would rather die than live—”
“Hush, Jane! you think too much of the love of human beings; you are too impulsive, too vehement.”
Helen's response here is Daria coupled with religious fervor while Jane's is pure passion and anxiety. Both are entirely appropriate to the time period. Jane, Helen, and Miss Temple do nothing to risk Jane or Helen actually being expelled.
In sum, in believable contemporarily written historical novels, characters are aware of the real risks of their setting and try to handle them.
The characters are mostly all members of the gentry, but unlike the others, Guy and Amanda are reliant on a stingy and censorious aunt for their allowance, Amanda's marriage prospects, and their overall acceptance in genteel society. A scandal would massively hurt that relationship.
Consequently, towards the end of the novel, Guy and Philip have a raging row about Guy bowing to "proprieties," but then, Philip can afford to shrug his shoulders at society while Guy is thinking about the future and survival. Guy and Amanda do want to be free of the aunt. They consider selling their books and furniture and setting up on their own. That decision would entail a total change in lifestyle for the siblings, which Philip doesn't understand until some of his more Bohemian friends point it out to him.
When Philip returns to clear the air and offer Guy a better alternative, Guy still doesn't thumb his nose at his aunt. He is as courteous as he can be. As Philip states,
"[M]erely listening to that conversation [between you, Guy, and your aunt] just now, I wanted to leave that room a pile of smouldering wreckage. Whereas you didn't, despite far greater provocation. You thought; you were kind; you at least made a reconciliation possible in the future...I'm already notorious. I can't prevent any of that, so the question is whether you are prepared to accept a certain amount of unwanted attention of ill-natured gossip [by becoming Philip's secretary]."
Guy replies,
"I daresay I can learn not to mind being talked about, if you don't mind trying to be talked about a bit less."
The language is somewhat modern. But the acknowledgment of real choices and real consequences and real risks within the milieu makes the book "historical."
Capturing the mindsets of the past may not be entirely possible--it should be attempted.
