If you look at the bottom of OpenWrt 21.02.1 First service release, you will see the notification channels that have been setup for this purpose.
You can receive release announcements via Discourse with the bell icon in the topic view for the Release and security announcement category (I use "Watching first post"), or you can sign up for the mailing list at https://lists.openwrt.org/mailman/listinfo/openwrt-announce.
If you really want machine-readable data, you should be able to consume the tagged releases via https://github.com/openwrt/openwrt/tags.atom.