I have read this sentence in a newspaper
Reality Winner pleaded guilty in June to a single count of transmitting national security information.
[Handout via Reuters]
Why is an adjective "guilty" following a verb "plead"?
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In this context, guilty is effectively a noun, the object of "plead". Historically it (or at least "not guilty" - "plead guilty" came later) was the name of a plea, so the phrase was really:
plead 'Not Guilty'
The noun can be used in a very limited set of contexts: beside "plead guilty" you can say "a verdict of guilty".