See the uBlock Origin author's post: https://github.com/uBlockOrigin/uBlock-issues/issues/338#iss...
This is ironic, because uBlock implements an extremely efficient filter and is even looking into using WASM to speed it up even more. Google's public position is that implementing functionality in JS or WASM is unacceptably slow. They say "[Preventing or weakening ad blockers] is absolutely not the goal. In fact, this change is meant to give developers a way to create safer and more performant ad blockers."[1]
Google's public position is also that WASM is "consistently fast"[2], fast enough to rewrite Google Earth to target it[3], and "It's entirely feasible to build a complex code-base to run performantly in the browser using WebAssembly"[4].
So which is it? Is the Web Request API being deprecated because it's not possible to write performant code in extensions using Chrome's powerful JS and WASM engine, or is it possible but there might be some other, different reason that they're blocking it?
[1] https://blog.chromium.org/2019/06/web-request-and-declarativ...
[2] https://developers.google.com/web/updates/2019/02/hotpath-wi...
[3] https://blog.chromium.org/2019/06/webassembly-brings-google-...
[4] https://developers.google.com/web/updates/2018/08/wasm-av1#f...