The usage over time is interesting, according to Google:
It spiked in the early 1800s before growing in popularity from the 1970s until now.
Even for non-language-purists, there is a right and a wrong way to use the word. We should use it in place of 'himself' or 'herself' when the gender is unclear. We shouldn't use it to replace 'themselves'.
Right: A person who scores an 8.0 on the IELTS test may well feel proud of themself.
Wrong: They all did well on the IELTS test and they felt proud of themself. (should be themselves)
