The Cerious Software development cycle is notoriously slow. ThumbsPlus v10 was released years ago, and v11 has been "in development" for a long time. There is no official roadmap for deep WebP support, including:
Your version lacks the encoder. Upgrade to ThumbsPlus v11 or use the libwebp plugin method described in Part 4. Alternatively, save as PNG/JPEG first, then use a free tool like cwebp (command line) post-process. thumbsplus webp
If you are using ThumbsPlus (a popular image database and thumbnail generator), here is everything you need to know about getting WebP files to work seamlessly. The Cerious Software development cycle is notoriously slow