pamfixtrunc was replaced in Netpbm 10.66 (March 2014) by pamfix.
pamfix with a -truncate option is the same thing as pamfixtrunc. But pamfix has other options to repair other kinds of corruption.
Another change that came with Netpbm 10.66 is that an invalid sample value (a value greater than the maxval the image declares in its header) is considered by the common Netpbm image reading facility (in libnetpbm) to be unreadable, which means the file is essentially truncated. In older Netpbm, the invalid sample value propagates to the output in a program such as pamfixtrunc. Thus, in older Netpbm a file with 100 rows and an invalid sample value in the 3rd row would pass through pamfixtrunc unchanged. But in Netpbm 10.66, pamfix -truncate with the same input would produce an output image with only 2 rows. While it is not possible in 10.66 to cause pamfix to generate an invalid Netpbm image, you can use -clip and -changemaxval options to avoid truncating the file in a case like this.
You should not make any new use of pamfixtrunc and if you modify an existing use, you should upgrade to pamfixtrunc. But note that if you write a program that might have to be used with very old Netpbm, pamfixtrunc is the only way to do that.