Issue
You would like to use unlogged tables on Heroku Postgres.
Resolution
Creating unlogged tables on Heroku Postgres (CREATE UNLOGGED TABLE) is heavily advised against. Unlogged tables are not crash safe, and will be truncated in the event of a crash or otherwise unclean shutdown, losing all data in the unlogged table in the process.
Additionally, as they are not written to the write-ahead log, they are not covered by Continuous Protection (ref: https://devcenter.heroku.com/articles/heroku-postgres-data-safety-and-continuous-protection).
Finally, as a consequence of not being written to the write-ahead log, unlogged tables are not replicated to followers or high-availabiltiy standbies.
This makes data recovery in unlogged tables impossible in the cases of maintenance operations, as well the aforementioned crashes and unclean shutdowns.