I wouldn’t strike out any one kind of language in fiction and the same is true for formal words like those you single out in your question. Still, like all other language choices, formal words should be used when appropriate and necessary to the story.
Ethan Canin uses formal language beautifully in his novella “The Palace Thief.” The narrator, William Hundert, is a classics teacher at a boys’ school and formal language is authentic for this character. The story begins like this:
I tell this story not for my own honor, for there is little of that here, and not as a warning, for a man of my calling learns quickly that all warnings are in vain. Nor do I tell it in apology for St. Benedict’s School, for St. Benedict’s School needs no apologies. I tell it only to record certain fortellable incidents in the life of a well-known man, in the event that the brief candle of his days may sometime come under the scrutiny of another student of history.
If you try to use formal language for a fifteen-year-old narrator attending public school in Brooklyn or a kindergarten teacher interacting with her students, you’ll be stretching the bounds of authenticity. Even if you’re not using first person—where the character is speaking directly to the reader—you want to be careful with a more formal voice. Depending on how you use it, a formal voice can sound regal, educated or pompous. If that isn’t serving a purpose or creating a specific effect for the reading experience, it could be off-putting.
Embrace your own tendencies in writing and cultivate your unique authorial voice, but always consider the impact of your choices.