Postgres 17 — anything that matters for application developers?

A few that are visible from application code:

• JSON_TABLE — turn JSON into a relational rowset directly in SQL.
• MERGE ... RETURNING — MERGE can now return the affected rows.
• Operational: incremental backups with pg_basebackup --incremental, and lower vacuum memory usage.

Nothing I know of forces app-level changes for a typical ORM-backed app, but run your test suite against 17 before switching — that is the real answer for your schema.