No, it is not. It is, I think, used to restrict the use of kanji in government documents/textbooks, but the joyo list is not legally binding for publishers and they only tend to use it as a guide. That is why things like the 新聞漢字表 exist - they are internal guidelines which differ from the joyo list, which newspaper publishers use. I'm not picking out those 18 kanji at random - those are the kanji that one particular (major) newspaper has picked as useful, and they're all Japanese. 炒める (stir-fry) and 栗 (chestnut) were both in my original list of useful non-joyo kanji I could think of, because you see them a lot in cooking/gourmet articles. 絆 has been pretty popular as a term recently, too. I recognise some of the others - not all, but they might be useful in specific contexts. They're not "chance of occurrence negligible", because they wouldn't bother to even come up with the list if they weren't using them regularly.
Here's wikipedia for you:
As the aim of the joyo list is to provide a standard for kanji use, not a restriction,
it has no force in law.
珍しい中国・東北部の家庭料理とは : グルメが何でぃ : フード&スイーツ : グルメ : YOMIURI ONLINE(読売新聞)
- here's yomiuri putting furigana in brackets for some words (placenames, chinese terms), but not for 片栗粉 炒め 水餃子 or 醤油. Are you telling me they just forgot? No, it's because those terms are recognizable to the sort of people who read the gourmet section of the newspaper, so they don't need furigana.
Joyo coverage for
what? It depends hugely on what you're reading.
菅氏「小沢氏の個利個略だ、呪縛から離れよ」 : 政治 : YOMIURI ONLINE(読売新聞)
In the first sentence of this (first thing on the front page, blergh, politics), there are no no-joyo outside names in the first sentence.
There are also no non-kyouiku outside names in the first paragraph
In the second (still ignoring names) there are four non-kyouiku, but joyo kanji: 離 and 縛, 呪 and 駒 (last two are on the new list)
In the third (just one sentence) there is one 及 which is not in the kyouiku but is in the joyo.
In the last there is 摘 and 陣, also 狙 (new list)
106 unique kanji in 354 characters
菅前小沢郎田 (6) only used in names, so 100 kanji used in words.
92 of those are within the 1006 教育漢字 (kyoiku).
8 of those are joyo but not kyoiku (half of them from the new list).
So yes, the joyo would cover everything in this particular document, but you don't need to learn over a thousand extra characters to pick up just those 8, do you? You can't say anything from this about how many of those joyo characters you
wouldn't need to bother with to be able to read this, or . And I would argue that if you can't figure out the meaning of 指摘 from the context and some knowledge of 指, you're doing something wrong, so it's not as if it's as simple as "percentage of kanji you know in this article = percentage understanding of this article".
Let's look at the second and third paragraphs of the foodie article I linked above, for a total of 425 characters. It contains 84 unique kanji excluding terms which had furigana attached, which I have removed.
63 (75%) are kyoiku
14 (17%, rounding up) are joyo, non-kyoiku.
So 92% are joyo (one was new-list).
8% are outside the joyo list (栗 炒 醤 鴨 酥 餃 瓜).
That the non-joyo all about food/cooking is not surprising. This will be the same for any specialist subject.
All I'm saying is that really, it's more efficient to go through something like the kyouiku kanji and then focus on vocabulary (deeper knowledge, for fewer kanji), picking up more kanji where appropriate depending on what you're reading, then to go through all the joyo and then figure out which ones were useful afterwards.
I'm speaking as someone who worked through Kanken DS and similar, and then chose to stop doing them because the vocabulary that it was requiring me to learn was too far removed from the vocabulary I wanted to learn. I did most of the food-themed kanji quizzes on "なぞっておぼえる大人の漢字練習" instead, that was far more useful (to me) then grinding through the joyo list for no particular reason.