@dilmandila @EverydayMoggie I mostly use full plural forms, but have developed an affection for "themself" in place of "themselves".
Regarding the dropping of thou from English (barring regional dialects that still use form of it), the plural "you" was also a mark of respect and over time "thou" became regarded as rude.
You can see an opposite kind of thing happening in (online only!) French, where the plural/respectful "vous" is rarely used and even strangers are regularly addressed with the familiar "tu".