Based on the feedback from the community, and the extra information we’ve collected about the effect on Windows 7/8, we’ve decided to delay this change until after the next Discourse stable release in July 2025. That will give communities and users another 3 months to prepare for the change.
That also give self-hosted administrators the option to switch their communities to the stable branch, which will continue working on older browsers until the following release at the beginning of 2026.
To enable us to keep pushing forward with new technologies, our new “Horizon Theme” is already using some of these modern browser features. For sites running Horizon, users of old browsers are already being shown the basic-html view.
I’ll update the OP here accordingly 