TimescaleDB, because it is packaged as a PostgreSQL extension (and not a fork, unlike the others), stays compatible with mainline PostgreSQL, especially as PostgreSQL improves. This is one of the key advantages of our approach.
(Timescale co-founder)
We don't yet know how we're going to do major version migrations, as the product is still not even out of private beta.
Customers don't always want to upgrade to new version when old versions just work. However this can lead to version creep. Ideally we want to always run the latest version of Postgres. We will hold this line as long as possible.
With separate storage and compute it seems tantalizingly possible to have low-effort zero downtime upgrades for the user, which would be a huge selling point.