Is OrioleDB interested in committing to a stable on-disk format removing the need for an upgrade process between Postgres major versions? Seems like an opportunity to solve this problem.
The need for the PostgreSQL upgrade process doesn't generally arise from the low-level on-disk formats of Postgres' heap and OrioleDB's table access method, but from changes in Postgres' catalogs. Things like the addition of a new type and its support functions will need to be inserted by some upgrade process. Then there are other catalog changes that change the column layout of the catalog tables, which also requires a process to update the stored data between the versions.
Without an upgrade process, you cannot change the catalogs, which is why only minor version upgrades of PostgreSQL can be done with only the swap of a binary, and can be rolled back safely without issue. It would limit upgrades to only internal APIs, planner, and executor changes, which would severely limit development.
I doubt that OrioleDB would be able to remove this need for an upgrade process for you.