Notes from a recent meeting of the Theme Review Team reflect a discussion of what methods for upselling are currently allowed in the theme repo. A majority involved in the discussion so far are in favor of enacting some restrictions, which include:
No global nags at top of admin pages. +12
One top-level link in the customizer (other unobtrusive links in sections allowed). +8
Allow one Appearance sub-page. +11
No options or panels/sections behind a paywall. +11
I'm all for a reasonable control (I think the above suggestions are reasonable), but I can imagine some push back on this.