While people generally use both for all intents and purposes and for all intensive purposes to mean “in every practical sense,” ”seeming as if,” or “virtually/almost completely,” the standard form of the idiom is for all intents and purposes.
“For all intents and purposes, Ethereum Classic is a worthless scam.”
Huffington Post“My mother’s heart would be broken to know her siblings have, for all intents and purposes, cut me out of their lives.”
Chicago Tribune“Your money, for all intents and purposes, is infinite; there are millions of ways to make money.”
Huffington Post
It may not pass the editorial process but it is not uncommon for you to encounter the use of for all intensive purposes both in speech and in print.
“For all intensive purposes, the Martellus Bennett era in Chicago is officially over.”
FanSided
The confusion may be attributed to the phonetic similarity between these phrases. If spoken aloud, for all intents and purposes and for all intensive purposes would virtually sound the same. This is why for all intensive purposes is considered a fairly common eggcorn, a word or phrase that result from a mishearing or misinterpretation of another.
For all intents and purposes, on the other hand, originated from the idiom “to all intents and purposes” which was used in English law in the 1500s. It is a shorter form of the original phrase “to all intents, constructions, and purposes” found in an act adopted by Henry VII in 1547.
The use of “for” in place of “to” eventually gained popularity, hence, the emergence of the idiom’s current form. However, “to all intents and purposes” is still being used by some writers.
“To all intents and purposes, bitcoin is ‘discovered’ by computers that compete globally with one another to gain credit for sealing off the latest block in the chain.”
Forbes
In conclusion, you are advised, for all intents and purposes, to use the correct idiom and avoid using the eggcorn especially in your written work.