How to fix broken text to speech rate on Big Sur
Well, it looks like it is a regression from macOS 10 to macOS 11. I confirm your saying, it worked and now Siri's voice speed can't be controlled anymore in Preferences Pane &gt; Accessibility &gt; Spoken Content
Apple does not notify anything about it in macOS 11's doc
The feature seems to be broken and bugs about Spoken Content are not new on MacOS. Look at this for instance.
Unfortunately, I think your only option is to open a bug ticket on Apple's website and wait until enough people are complaining about it to get it fixed.