I have an observation on the original use of OPML, i.e., for outliners (the one I utilize frequently) : Many recent outliners (logseq, roam) added custom properties/fields (for DB-like block-level queries), but they don't seem to support them when importing or exporting opml. The only "field" that is somewhat supported is the "_note" [1].

[1] https://web.archive.org/web/20131205072742/https://jeffreyki...