Pbiviz version 4.0.8 was released about a week ago. In the changelog, it's noted this is a reversion to a stable version since some critical bugs were discovered in version 4.0.7. I've downloaded, tested, and confirmed that interactive R (specifically plotly) custom visuals still do not work in this latest version of pbiviz. You can create an rhtml template folder and successfully package it as a custom visual, but it will simply not render/appear blank when imported into a power bi report.
In case other new power bi users are struggling with this issue, pbiviz version 4.0.5 is the latest version I've found where the rhtml template works as intended. This issue is not isolated to a single user (me) as I've seen the problem described by at least two other users, linked below.