The CSS Speech Module Level 1 (CR draft, 14 February 2023) speak property is now flagged as "Unrecognized or unsupported property at line x character y." by css-sanitizer when attempting to save a sanitized-css page containing it.
The case at hand is s:Template:Header/styles.css. If I just edit that page and immediately save it I get the error message, because the speak property apparently worked on 22 January 2021 when it was first added to the page.
On pages with CSS content model (like [[Special:MyPage/common.css]]) it works fine. But any TemplateStyles page (i.e. sanitized-css) I tested I got the same behaviour.
Now admittedly my codesearch-fu isn't all that, but as best I can figure out this is due to rCSSSe3dc606034acc31771ffbb5d13d11bf84f2bb39f by @Anomie on 6 February 2022. According to the commit message that commit drops CSS Speech entirely because it is "obsolete", which I think must be a mistake. The old (2012) CSS Level 3 Speech Module was retired in 2018, but that was just the normal diffusion of CSS specs to replace it with CSS Speech Module 1 (which was current from 10 March 2020). At time of writing the 14 February 2023 CR draft is current, and just skimming the two versions suggests this is indeed just a revision rather than a completely new spec.