Tractatus Logico-Philosophicus, Passages from the Life of a Philosopher, and A Tangled Tale are probably our most MathML-heavy books.
Books from the PD era are not very likely to feature math serious enough to require MathML. This is probably for the best... MathML support in ereaders is poor, with the exception of iBooks and Kobo. Raw MathML is retained in our "advanced" epubs, but it's converted to PNGs in our "compatible" epubs, for this reason.
https://standardebooks.org/ebooks/ludwig-wittgenstein/tracta...
https://standardebooks.org/ebooks/charles-babbage/passages-f...
https://standardebooks.org/ebooks/lewis-carroll/a-tangled-ta...
It’s certainly not “serious math,” but I’m rather proud of this revision I made to the MathML used for a throwaway equation in an obscure short story: https://github.com/standardebooks/fritz-leiber_short-fiction...
Indeed, in academic writing I never see MathML used in an HMTL setting, it's always MathJax or KaTeX. For your purposes this is probably fine, but imagining if someone wanted to author a high quality math textbook following the same standard I would wager they'd run into a brick wall