The phrase "confident of" is correct and can be used in written English.
It is used when expressing confidence in something or someone. For example, "I am confident of my abilities to produce quality work."
Exact(59)
"I'm confident of that.
"We're confident of that".
I am confident of that".
KVAM is confident of victory.
He seems confident of victory.
We remain confident of regulatory approval".
Reds manager confident of keeping his star.
We remain confident of earning regulatory approval".
Not everyone seemed confident of a win.
She is very confident of her ability.
Mr Clark is, naturally, confident of success.
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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