Native logical replication (making it possible to upgrade without downtime) was introduced in pgSQL 10. But if you're going to have downtime anyway, there's no reason not to do the upgrade in a single step. pg_upgrade should support that quite easily.

also, logical replication has some issues that make it not necessarily perfect for cross version migration: schema changes and sequence usages are not synced.

While schema changes are probably not much of a problem because that's something you can easily prevent during migration, sequence usage is because once you fail over to the logically replicated secondary, you will have to reset all sequences to their current values or all further inserts into tables with sequences will fail.

the other option, of course is to not use sequences but rather use UUIDs for your primary keys, but those have their other collection of issues

Because version below 10 don't support logical replication. The alternative would be to use a 3rd party extension for replication, which may well be a good option but I don't have any experience with that so I can't really comment.