The phrase "in form" is correct and usable in written English.
You can use it to indicate that something is in the shape or structure that is expected or required. For example, "The essay was handed in on time, and it was in form."
Dictionary
in form
adjective
Having good form; having good recent performances.
Exact(55)
He is also in form.
Metamorphism means change in form.
Plainly he's interested in form.
His outgoing personality is already in form.
Story is in form of a log.
In form, Hamlet is a revenge tragedy.
Similar(5)
Lewandowski is one of Europe's in-form players and probably the world's in-form striker.
The new man is the in-form George Bailey.
She comes from the in-form Jonjo O'Neill yard.
He's our in-form batsman.
Newcastle are an in-form side.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com