The American Standard Code for Information Interchange is a 7-bit character encoding. Work on the standard began in 1960 and the first edition was published in 1963. The standard attempted to include characters which were common in other character encodings (such as EBCDIC) and equipment (e.g. typewriters, printers, teletypes) at the time.
Which characters/symbols/glyphs which were in use by other encodings/equipment in 1963 or earlier didn't (eventually) make it into ASCII?
Because ASCII is an "American" code, non-English letters don't count. Nor do things that were added in later versions of ASCII (e.g. curly brackets).
Examples: The cent sign ¢ was common on many typewriters and was bit pattern 100111 in the FIELDATA character set used by UNIVAC and Unisys.
The square lozenge ⌑ was part of the first BCDIC character set in 1932, and continued to be used in its successor EBCDIC. It was also used in the IBM 026 punched card code.