The lack of a table of contents in Xaringan has actually been an open issue since 2019. Other html presentation formats in R Markdown, such as revealjs::revealjs_presentation and slidy_presentation do provide a table of contents, which allows for a faster navigation of the slides.
As Yihui explained in the issue page above, an implementation of a table of contents for Xaringan is actually a simple project requiring JavaScript. So, I've implemented just that, respecting the properties explained by Yihui in this comment of his.
Please visit this link in order to see a demo of this feature.