As I believe your intuition says, "are things of the past" is much more common than "are a thing of the past". Consider the Google Ngram below:
And looking at the actual citations, a reasonable fraction (at least a quarter of them) are grammatical constructions parallel to "to-do lists are things of the past".
However, Google still finds many, many examples parallel to "to-do lists are a thing of the past". These are grammatically correct because in the construction A is/are B, English makes no requirement that A and B have the same number; the requirement is that the verb agrees with A. For a different example, I think that native English speakers would be more likely to say "to-do lists are a problem" than "to-do lists are problems". You can use Ngrams to check that this is currently true (it's changed from 50 years ago) if you replace to-do lists with men, women, or people.