There are some specific IFSC codes that are mentioned on the NACH Page: https://www.npci.org.in/national-automated-clearing-live-members-1, and do not show up anywhere on the RBI website.

IMPS/ACH is supported on these IFSC codes, but NEFT and RTGS are not. A complete list of branches that only show up on this page, but not on RBI website:

APBL0019001  ALLA0888888  HDFC0CAMSBA  KKBK0BUCB01  CRLY0000001  CSBX0CSB001  APBL0014001
APBL0005001  APBL0022001  FDRL0000379  SBIN0000DOP  APBL0015022  YESB0HPB051  UTIB0SIDB99
IOBA0009016  HDFC0CITC01  CBIN0MPDACT  GSCB0JNDC01  INDB0KUCB01  APBL0020001  HDFC0CLCBLO
MAHB0003007  APBL0017001  UTIB0STMNBI  ICIC00NDIMC  APBL0023001  YESB0NBGKPI  UTIB0SBCUB1
RBIS0RBIPI0

However, the above page only lists 1 IFSC code per Bank, which leads me to wonder: are there are any such branches. If there are, an exhaustive list of all ACH IFSC codes will be great to have.