Even more concise:

data = data[:75]

If it is less than 75 characters there will be no change.

Subclassing str and overriding the __format__ method is an option:

class CustomStr(str):
    def __format__(self, spec):
        if spec == 'trunc_left':
            return self[-8:]
        else:
            return super().__format__(spec)

git_sha = 'c1e33f6717b9d0125b53688d315aff9cf8dd9977'
s = CustomStr(git_sha)

print('{:trunc_left}'.format(s))

Better though, you can create a custom Formatter which inherits from string.Formatter and will provide a format method. By doing this, you can override a number of methods used in the process of formatting strings. In your case, you want to override format_field:

from string import Formatter

class CustomFormatter(Formatter):
        
     def format_field(self, value, format_spec):
         if format_spec.startswith('trunc_left.'):
             char_number = int(format_spec[len('trunc_left.'):])
             return value[-char_number:]
         return super().format_field(value, format_spec)

environ = {'git_sha': 'c1e33f6717b9d0125b53688d315aff9cf8dd9977'}
fmt = CustomFormatter()
print(fmt.format('{git_sha:trunc_left.8}', **environ))

Depending on the usage, you could put this in a context manager and temporarily shadow the builtin format function:

from string import Formatter

class CustomFormat:
    
    class CustomFormatter(Formatter):
        
        def format_field(self, value, format_spec):
            if format_spec.startswith('trunc_left.'):
                char_number = int(format_spec[len('trunc_left.'):])
                return value[-char_number:]
            return super().format_field(value, format_spec)
            
    def __init__(self):
        self.custom_formatter = self.CustomFormatter()
            
    def __enter__(self):
        self.builtin_format = format
        return self.custom_formatter.format
        
    def __exit__(self, exc_type, exc_value, traceback):
        # make sure global format is set back to the original
        global format
        format = self.builtin_format

    
environ = {'git_sha': 'c1e33f6717b9d0125b53688d315aff9cf8dd9977'}

with CustomFormat() as format:
    # Inside this context, format is our custom formatter's method
    print(format('{git_sha:trunc_left.8}', **environ))

print(format)  # checking that format is now the builtin function