Preposition

I would say:
I've attached the requirements for this project for which you had provided the plans.

'Had' implies 'previously' - redundant

The general rule (which is likely going out the window now) is you don't end sentences with a preposition. So many of the old school grammar rules are no longer being followed, but if you are writing something for school or business, they should still be followed. If you don't, you sound illiterate. People do notice.

It is poifect.

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That "rule" was never a rule. It was invented by John Dryden back in the days when all these totally nutty rules that you mention were invented out of whole cloth. One can tell that they were never rules of English because the users of English NEVER followed them, before they were invented or after because they were not ever part of the grammar of English. Not even any of the best writers in English followed them.

Thank you!

I've attached the project requirements for your plans.

How many months, years have you had to bring yourself up to speed/reality regarding these "rules" that have never ever been real rules of grammar?
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To whom I say: Maven, shmaven! [Kibbitzers] and [nudniks] is more like it. For here are
the remarkable facts. Most of the prescriptive rules of the language mavens make no
sense on any level. They are bits of folklore that originated for screwball reasons several
hundred years ago and have perpetuated themselves ever since. For as long as they have
existed, speakers have flouted them, spawning identical plaints about the imminent
decline of the language century after century. All the best writers in English have been
among the flagrant flouters. The rules conform neither to logic nor tradition, and if they
were ever followed they would force writers into fuzzy, clumsy, wordy, ambiguous,
incomprehensible prose, in which certain thoughts are not expressible at all. Indeed, most
of the "ignorant errors" these rules are supposed to correct display an elegant logic and an
acute sensitivity to the grammatical texture of the language, to which the mavens are
oblivious.

The scandal of the language mavens began in the 18th Century.

READ ON AT,

https://homepages.wmich.edu/~hillenbr/204/GrammarPuss.pdf