I was wondering the same. You can see a lot of new functions and things happening on plotly.js and plotly for python. But R plotly looks the same in the last 2 years.

The version of plotly.js that is bundled with the R package can be updated by running this script https://github.com/plotly/plotly.R/blob/master/tools/update_plotlyjs.R

I'm typically pretty conservative about updating plotly.js; but that said, it has been almost a year since the last update (#2096), so I'd be open to a PR that does a version update.