Though I do notice that introducing these will leave the HTML-to-markdown converter unaware of these attributes. But as demonstrated above, markdown-it currently can’t handle markdown syntax representing these anyway. For that, it may be desirable to go a step further and implement list types in CommonMark…
The lack of this makes it very difficult to quote official documents, legal documents, software licenses, or anything else which use non-number markers for document references.
That’s a good solution if the goal is arbitrary list decorations, but the HTML type attribute is specifically meant for semantically-meaningful situations where the difference between list markers are more than decorative. From <ol>: The Ordered List element - HTML: HyperText Markup Language | MDN
Unless the type of the list number matters (like legal or technical documents where items are referenced by their number/letter), use the CSS list-style-type property instead.
It’s a common enough need that although the HTML attribute was deprecated in HTML 4.01 (with justification of it being purely stylistic), it was subsequently re-introduced in HTML5 for the specific purpose of encoding reference lists as described by MDN.