Strike through plain text with unicode characters?

There's U+0336, COMBINING LONG STROKE OVERLAY. Googling Unicode strikethough turns up tools to apply it to your text, such as this one. Whether it looks any good will depend on your font; it looks pretty terrible on my environment.

You can also use this function in Python 3+ to generate it on the fly:

def striken(text):
    return ''.join(chr(822)+t for t in text)

Example output

>>> def striken(text):
...   return ''.join(chr(822)+t for t in text)
... 
>>> striken("hello")
'̶h̶e̶l̶l̶o
>>> striken("hello darkness my old friend")
'̶h̶e̶l̶l̶o̶ ̶d̶a̶r̶k̶n̶e̶s̶s̶ ̶m̶y̶ ̶o̶l̶d̶ ̶f̶r̶i̶e̶n̶d'

Also you may use:

def striken(text):
    return '\u0336' + '\u0336'.join(text)

to be faster if your text is really long, as suggested by @astroMonkey.