I can only speak for myself, but as KOA mentioned, I only need a matter controller, not a Thread , Zigbee-wireless (or even WiFi-) access point.
I agree if the wireless chip supports these wireless protocols that there is no reason to not implement these in OpenWRT (apart from human resources).
For Zigbee users, a Zigbee-coordinator plugin would be useful in this case to avoid needing an extra ZigBee hub or zigbee-lightbulb (or whatever zigbee router device) that has to be kept online the whole time.
A secure implementation is mostly preferred, but again I speak only for myself, my current non-smart (light) switches, shutters, heating controls, fridge, sound system, TV, even my PC are absolutely not secure. Everyone who enters my house is able to do whatever they want with them. I would be as happy with a 'non-secure' solution, especially since I'm not planning to expose these to the internet. I think this applies to >90% of the users.
Of course , if a matter controller is as resource needing as Home Assistant , there is no real point in implementing one on OpenWRT (HA can already be run on OpenWRT on rather powerful hardware).
But is that actually the case? I've not dived deep in the hundreds of pages Matter specification documentation. But my understanding of a matter controller is a server that accepts XML or JSON like messages (containing device id, cluster, action/event, and value, e.g. ab24e1f , lighting, setbrightness, 50) and sends it to the intended device (who actually handles the Matter-message). Hence I assumed most low-end home router hardware qualify to run this smoothly. But maybe I have a too simplistic view on it