This is the first byte of the ELF file.
Linux has a system of "magic numbers" that are use to recognise specific file types. Apart from being helpful for the
file utility, they are used by the kernel to recognise executables.
The 'magic' concept has a long history, but there has been little or no correlation on specific values between either manufacturers (such as Sun, Dell, HP etc), or package creators.
Using any initial printable character (such as E) could be plain text. Using 0x80 (octal 0200) and above could mark a UTF-8 multi-byte character. Many of the ASCII control chars would be problematical (NUL, NL, TAB). There is not a great number of options. So the initial 0x7F (DEL) byte is a reasonable choice to prevent any such confusion.
You could look at your local man pages for
It is actually no accident that DEL is 0x7F. When data was often held on paper tape, a mis-punch could be obliterated by punching out all the chads (plus the 8th bit, for even parity). Most readers skipped the NUL and DEL characters completely.