The original TSM engine is still used by InfluxDB v2 OSS.
The InfluxDB Cloud platform uses a variation of TSM that's tailored for a distributed SaaS rather than stand-alone nodes (this was originally intended to be used in InfluxDB v2 OSS as well, but alpha-testing showed that the old engine performed better there so it ultimately was reverted for the beta release).
So IOx is really the first major new storage engine in InfluxDB.
The InfluxDB Cloud platform uses a variation of TSM that's tailored for a distributed SaaS rather than stand-alone nodes (this was originally intended to be used in InfluxDB v2 OSS as well, but alpha-testing showed that the old engine performed better there so it ultimately was reverted for the beta release).
So IOx is really the first major new storage engine in InfluxDB.