I finally got curious enough to script it. Some basic stats about the non-deprecated exercises on multiple live tracks.
| Exercise | Difficulty_count | Difficulty_median | Difficulty_mean | Difficulty_stddev |
|---|---|---|---|---|
| hello-world | 78 | 1 | 1.051 | 0.222 |
| leap | 78 | 1 | 1.333 | 0.526 |
| two-fer | 74 | 1 | 1.284 | 0.511 |
| accumulate | 17 | 2 | 2.059 | 1.088 |
| acronym | 77 | 2 | 2.688 | 1.310 |
| armstrong-numbers | 74 | 2 | 2.365 | 1.105 |
| collatz-conjecture | 77 | 2 | 2.221 | 1.119 |
| darts | 73 | 2 | 2.041 | 0.841 |
| difference-of-squares | 76 | 2 | 1.842 | 0.767 |
| eliuds-eggs | 63 | 2 | 2.381 | 0.888 |
| etl | 67 | 2 | 2.657 | 1.286 |
| gigasecond | 54 | 2 | 2.056 | 1.235 |
| grains | 72 | 2 | 2.306 | 1.057 |
| hamming | 76 | 2 | 2.132 | 1.011 |
| high-scores | 49 | 2 | 2.490 | 1.416 |
| isogram | 76 | 2 | 2.408 | 1.110 |
| line-up | 23 | 2 | 2.609 | 0.722 |
| micro-blog | 19 | 2 | 2.211 | 0.787 |
| nucleotide-count | 69 | 2 | 2.522 | 1.079 |
| pangram | 76 | 2 | 2.263 | 0.915 |
| raindrops | 78 | 2 | 2.051 | 0.924 |
| resistor-color | 65 | 2 | 1.662 | 0.756 |
| resistor-color-duo | 64 | 2 | 2 | 0.756 |
| resistor-color-expert | 3 | 2 | 1.667 | 0.577 |
| reverse-string | 73 | 2 | 1.767 | 1.264 |
| rna-transcription | 76 | 2 | 1.816 | 0.725 |
| space-age | 74 | 2 | 2.122 | 1.134 |
| square-root | 56 | 2 | 2.589 | 1.141 |
| sum-of-multiples | 69 | 2 | 2.391 | 1.331 |
| triangle | 74 | 2 | 2.405 | 0.905 |
| trinary | 3 | 2 | 2.333 | 0.577 |
| anagram | 74 | 3 | 3.419 | 1.596 |
| binary | 6 | 3 | 2.667 | 1.033 |
| binary-search | 72 | 3 | 3.403 | 1.401 |
| bob | 75 | 3 | 2.680 | 1.296 |
| bottle-song | 45 | 3 | 3.511 | 1.272 |
| clock | 54 | 3 | 3.444 | 1.208 |
| dnd-character | 50 | 3 | 3.120 | 1.172 |
| error-handling | 10 | 3 | 3.200 | 0.919 |
| isbn-verifier | 58 | 3 | 3.259 | 1.085 |
| kindergarten-garden | 61 | 3 | 3.574 | 1.335 |
| perfect-numbers | 65 | 3 | 2.923 | 0.989 |
| phone-number | 71 | 3 | 3.239 | 1.224 |
| prime-factors | 62 | 3 | 3.258 | 1.267 |
| protein-translation | 69 | 3 | 3.290 | 1.384 |
| proverb | 56 | 3 | 3 | 1.250 |
| queen-attack | 62 | 3 | 3.274 | 1.651 |
| resistor-color-trio | 52 | 3 | 2.904 | 1.287 |
| rotational-cipher | 68 | 3 | 3.294 | 1.247 |
| scrabble-score | 75 | 3 | 2.653 | 1.225 |
| secret-handshake | 72 | 3 | 2.986 | 1.216 |
| series | 57 | 3 | 2.982 | 1.408 |
| sieve | 72 | 3 | 3.681 | 1.471 |
| strain | 44 | 3 | 2.568 | 0.950 |
| sublist | 53 | 3 | 3.679 | 1.730 |
| all-your-base | 70 | 4 | 4.229 | 1.395 |
| allergies | 71 | 4 | 3.437 | 1.168 |
| atbash-cipher | 74 | 4 | 3.689 | 1.578 |
| bank-account | 32 | 4 | 4.281 | 1.871 |
| beer-song | 14 | 4 | 4.500 | 1.698 |
| diamond | 61 | 4 | 4.180 | 1.775 |
| flatten-array | 42 | 4 | 3.381 | 1.464 |
| grade-school | 50 | 4 | 3.680 | 1.269 |
| house | 45 | 4 | 3.600 | 1.355 |
| largest-series-product | 68 | 4 | 4.059 | 1.370 |
| list-ops | 53 | 4 | 4.302 | 1.804 |
| luhn | 71 | 4 | 4.085 | 1.637 |
| matrix | 51 | 4 | 3.353 | 1.036 |
| nth-prime | 65 | 4 | 3.800 | 1.641 |
| pascals-triangle | 65 | 4 | 3.923 | 1.350 |
| robot-name | 40 | 4 | 4.050 | 1.431 |
| robot-simulator | 62 | 4 | 4.097 | 1.523 |
| roman-numerals | 75 | 4 | 3.680 | 1.561 |
| saddle-points | 47 | 4 | 3.787 | 1.503 |
| simple-linked-list | 28 | 4 | 4.393 | 1.641 |
| swift-scheduling | 9 | 4 | 4.778 | 1.302 |
| twelve-days | 46 | 4 | 3.435 | 1.148 |
| word-count | 63 | 4 | 3.571 | 1.521 |
| yacht | 64 | 4 | 3.922 | 1.451 |
| affine-cipher | 40 | 5 | 4.675 | 1.542 |
| binary-search-tree | 38 | 5 | 5.079 | 1.477 |
| circular-buffer | 51 | 5 | 5.196 | 1.980 |
| complex-numbers | 31 | 5 | 4.871 | 1.857 |
| crypto-square | 51 | 5 | 4.706 | 1.858 |
| custom-set | 33 | 5 | 5.030 | 1.667 |
| diffie-hellman | 10 | 5 | 5 | 2.261 |
| dot-dsl | 8 | 5 | 5.125 | 1.808 |
| food-chain | 52 | 5 | 4.577 | 1.210 |
| game-of-life | 33 | 5 | 4.848 | 1.906 |
| grep | 18 | 5 | 5.222 | 1.478 |
| killer-sudoku-helper | 27 | 5 | 4.815 | 1.272 |
| knapsack | 59 | 5 | 5.644 | 1.517 |
| ledger | 9 | 5 | 5 | 0.500 |
| linked-list | 33 | 5 | 5.030 | 1.489 |
| markdown | 14 | 5 | 4.929 | 1.207 |
| matching-brackets | 72 | 5 | 4.208 | 1.635 |
| meetup | 56 | 5 | 5.018 | 1.711 |
| ocr-numbers | 36 | 5 | 5.361 | 1.869 |
| parallel-letter-frequency | 25 | 5 | 5.600 | 1.936 |
| pig-latin | 58 | 5 | 4.707 | 1.533 |
| pythagorean-triplet | 47 | 5 | 5 | 1.694 |
| rational-numbers | 25 | 5 | 4.960 | 1.207 |
| relative-distance | 13 | 5 | 5 | 0.408 |
| run-length-encoding | 65 | 5 | 4.215 | 1.754 |
| simple-cipher | 31 | 5 | 4.581 | 1.566 |
| spiral-matrix | 53 | 5 | 4.830 | 1.355 |
| split-second-stopwatch | 10 | 5 | 4.900 | 0.568 |
| state-of-tic-tac-toe | 27 | 5 | 5.148 | 1.167 |
| tournament | 30 | 5 | 4.933 | 1.172 |
| transpose | 35 | 5 | 4.543 | 1.559 |
| tree-building | 7 | 5 | 4.857 | 1.215 |
| variable-length-quantity | 35 | 5 | 5.229 | 1.767 |
| baffling-birthdays | 7 | 6 | 5.714 | 0.951 |
| bowling | 35 | 6 | 5.886 | 1.694 |
| camicia | 4 | 6 | 6 | 0.816 |
| change | 40 | 6 | 6.175 | 1.483 |
| flower-field | 48 | 6 | 5.729 | 1.498 |
| intergalactic-transmission | 15 | 6 | 6.133 | 0.743 |
| palindrome-products | 38 | 6 | 5.737 | 1.465 |
| piecing-it-together | 8 | 6 | 6.125 | 0.641 |
| rail-fence-cipher | 35 | 6 | 5.343 | 1.679 |
| say | 49 | 6 | 5.551 | 2.082 |
| two-bucket | 43 | 6 | 6.326 | 1.248 |
| word-search | 19 | 6 | 6.263 | 1.195 |
| wordy | 44 | 6 | 5.682 | 1.722 |
| alphametics | 23 | 7 | 7.087 | 1.703 |
| book-store | 35 | 7 | 6.429 | 1.614 |
| dominoes | 34 | 7 | 6.853 | 1.560 |
| minesweeper | 11 | 7 | 5.818 | 2.089 |
| paasio | 4 | 7 | 5.750 | 2.062 |
| poker | 23 | 7 | 6.783 | 1.476 |
| rectangles | 28 | 7 | 6.357 | 1.660 |
| rest-api | 11 | 7 | 6.818 | 1.079 |
| satellite | 18 | 7 | 6.722 | 1.565 |
| connect | 27 | 8 | 7 | 1.881 |
| forth | 28 | 8 | 7.821 | 1.765 |
| go-counting | 11 | 8 | 7.545 | 1.809 |
| hangman | 7 | 8 | 7.286 | 2.360 |
| mazy-mice | 2 | 8 | 8 | 0.000 |
| react | 17 | 8 | 8.412 | 1.326 |
| sgf-parsing | 14 | 8 | 8.071 | 0.997 |
| zebra-puzzle | 39 | 8 | 7.692 | 1.673 |
| zipper | 18 | 8 | 8.333 | 1.414 |
| lens-person | 5 | 9 | 8.400 | 0.894 |
| pov | 14 | 9 | 8.786 | 1.188 |
If you are curious, the source.