I believe they will explore using the new matrix-rust-sdk in Element Web further down the line, but it isn't a priority afaik. I'd love them to continue development of Hydrogen tho :p
Like rorschah said, it's a Matrix client, with a focus on being super lightweight and embeddable, having its own SDK. I enjoy it, since all other clients (with exception of iamb ig) use up a lot of memory on my account. Unfortunately, development has been very slow lately, and the client is missing important features like edits, text formatting and so on.
Yeah that's vodozemac, which is a Matrix crypto SDK written in Rust, and is part of the broader Rust SDK they are developing (which handles nesrly everything necessary for client development, even some opinionated UI stuff).