I have used pandoc before with the tufte.css to produce a bibliography with references in sidenotes and a full bibliography at the end; it put footnotes from markdown into sidenotes as well.
I now use citeproc with pandoc 3.1.8 and the bibliography notes are e.g. (Author-year) in the text. Footnotes inserted in markdown are also shown at bottom and not as sidenotes.
I think I still use the same tufte.css file as before. What did change? How to get the sidenotes back? Should I use pandoc-sidenotes, but perusing the code I doubt it would work with citeproc as it is produced now.
Alternatively, I could edit the csl bibliography style; I seem to remember some hints to change footnote to sidenote in the bibliography style on stackoverflow, but cannot find them again.
The citations can be moved to the side by adding .citation to the .sidenote class in the tufte.css file. The same trick does not work for the footnotes produced by markdown.