It sort of seems to me that Heroku stopped development / experiments after getting a bunch of traction. They were spot on solving a specific problem, early, and as time went on, said problem became a non-issue?
For a long time, they operated more or less like an independent org after the Salesforce acquisition. Eventually they’ve been pulled into the ecosystem though, and now it has the on-brand feel: 10+ second page loads, weird breakage like it taking two tries to click the SSO links to open addons like New Relic, needing to re-auth too often. It’s a shame.