This is mentioned in default delta.lst shipped with HL25:
DEFINE_DELTA( eflags, DT_INTEGER, 2, 1.0 ), // JoshA: Changed to 2 for EFLAG_FLESH_SOUND to fix flesh sounds on entities. If a mod ships updated DLLs and not this file, they just won't have that fix.
This is mentioned in default delta.lst shipped with HL25: