Interesting, I wasn't aware of that.

Checking manually, I found many of the low-numbered RFCs are present, though there are some higher in range which turn up 404.

E.g., RFC 1 is online: https://datatracker.ietf.org/doc/html/rfc1

But here's a list of missing HTML pages from the first 1024 RFCs:

8, 9, 14, 26, 51, 92, 159, 201, 220, 244, 248, 257, 258, 259, 260, 261, 262, 272, 275, 277, 279, 284, 337, 341, 358, 375, 380, 383, 397, 418, 424, 427, 428, 444, 465, 481, 484, 502, 507, 517, 530, 536, 540, 541, 554, 558, 564, 572, 575, 583, 598, 605, 639, 641, 646, 648, 649, 650, 664, 665, 668, 670, 673, 676, 682, 693, 709, 710, 711, 715, 723, and 853.

For anyone who wants to verify:

  wget --spider --force-html $(
    for i in {1..1024}; 
    do 
      echo "https://datatracker.ietf.org/doc/html/rfc${i}"; 
    done ) 2>&1 |
    egrep '(https.*rfc|awaiting response)' |
    grep -B1 '404 Not Found' |
    grep -v '404 Not Found'

Above RFC-2000 or so, the pattern of omissions starts to look more deliberate, suggesting reserved values:

8, 9, 14, 26, 51, 92, 159, 201, 220, 244, 248, 257, 258, 259, 260, 261, 262, 272, 275, 277, 279, 284, 337, 341, 358, 375, 380, 383, 397, 418, 424, 427, 428, 444, 465, 481, 484, 502, 507, 517, 530, 536, 540, 541, 554, 558, 564, 572, 575, 583, 598, 605, 639, 641, 646, 648, 649, 650, 664, 665, 668, 670, 673, 676, 682, 693, 709, 710, 711, 715, 723, 853, 1061, 1182, 1260, 1839, 1840, 3100, 3200, 3223, 3328, 3333, 3350, 3399, 3400, 3500, 3699, 3799, 3800, 3889, 3899, 3900, 3907, 3908, 3999, 4000, 4099, 4100, 4199, 4200, 4232, 4299, 4300, 4399, 4400, 4499, 4500, 4599, 4600, 4637, 4658, and 4699.