Wan Optimization is a collection of features. Some will accelerate encrypted traffic without decryption e.g:
Packet Loss Recovery & Reliable Stream Protocol
Forward Error Correction (FEC)
TCP Acceleration
Packet Aggregation / Packet Coalescing
Quality of Service
To apply these features to encrypted traffic, ensure that the application is not bypassed by default via the default encryption traffic policy.
The features above are often sufficient to accelerate encrypted traffic, but it is also possible to Cache, Compress and Deduplicate encrypted traffic when it is OK to Decrypt -> Optimize -> Re-Encrypt.
In the enterprise network, where the client is end to end responsible for the security of the HTTPS application, it is often acceptable. This is what Wanos focuses on in HTTPS caching and later deduplication+compression.
In the end user Internet scenario, where the ISP decrypts the end users HTTPS, that is quite a different scenario.