Google Ngram Viewer shows a decline in the use of “nothing loath” since the 1970s unlike its antonym “loath” which is still widely used.
Would it be appropriate for me to use it or has it become obsolete?
Learn English – Is the adjective “nothing loath” still in common usage
word-usage
Best Answer
You only need to look at one chart to answer the question. Note the variant spellings, which have declined in lockstep over the past century...
But I would also point out that at least some of the "contemporary" instances will actually be citing earlier usages, so the decline is in fact steeper than the chart would suggest.
Having said that, the expression is at least "known" to many native speakers. But that doesn't mean it's "current" in natural speech. On the rare occasions when people use it (in speech or in writing) today, it's usually somewhat "facetious" (effectively, a deliberate archaism).
Also note that it's a "set phrase" that doesn't reflect modern grammar. You could reasonably (if "mock-archaically") say "Our waiter was nothing loth to accept a handsome gratuity", but you certainly couldn't say he was "nothing unwilling", for example.