Yes you can repurpose your old ZBT-1 as a dedicated Thread Border Router for this if you want.
I have used it like so, but now I just ordered two ZBT-2 anyway to support the Home Assistant project and therefore will be migrating both my Zigbee network and my Thread network for Matter to two new ZBT-2 adapters 
I have done the same.
I could do that too, but already have the Dirigera providing me Thread devices.
is that the same as a thread border router? or would the ZBT 1 offer more in that respect
I have been moving all my current IKEA devices off the Dirigera. All of them are attached directly to Home Assistant via Zigbee. That used to be the ZBT-1 and worked great, but now use the ZBT-2. Most of the devices are bulbs, but even the blind works.
Yeah I am aware I can do that. However, I’d hate to lose my backup option of the ikea app in case of HA trouble…
One of the reasons I moved them over to Home Assistant was the Dirigera was unreliable and had to restart quite often. To be fair, the devices left on that side have been stable for a long time now. I wanted to get rid of hubs,
yeah, I get that.
I had endless troubles with the first generation Ikea hub, but the Dirigera has been rocksolid from day 1 to me.
All integrated HomeKit, and since a few months via Thread. for testing purposes, and because some of them have a few attributes / entities the Homekit does not provide.
Im just wondering now if the additional ZBT1 as thread border router can do additional tasks, or if the Dirigera will do.
The ZBT will probably be a bit more efficient on a usb, than the mains powered Dirigera
I have set up the new ZBT-2 and tried to add the Eve plug which is flash to be a Matter plug. I got this error on the iPhone in the companion app. I think it is due to this
The preferred network function isn’t completely implemented yet. In particular, when adding Matter devices through the companion apps, the preferred network of the mobile device is being used.
I have set Home Assistant as my preferred Thread network. Even thought I already have one, those devices will be rejoined or replaced so I decided not to merge the networks.
I will have to wait for the IKEA devices to come out before I can test it properly. The other Matter devices I have at WiFi, not thread.
FYI, sounds like IKEA have some of their new Thread-based products in stock now.
Did you confirm with @DriesA local IKEA store? 
IKEA Sweden at least list availbility and prices for all 21 new products the Swedish market:
Very nice prices too. Example
There is also an official press release for the launch
Here is a detailed list:
hello I just purchased and paired the new ikea myggsprayy motion sensor, details @ Ikea Myggspray -- matter over thread -- new motion sensor
here is a list from ikea with the new devices so far New products - IKEA
That is good news. It is not on the UK store as yet.
Did everything flawlessly?
I’m eager to hear some first-hand experience 
Remember that you also need to add many mains-powered Thread devices (which works as routers/repeaters) to the same Thead network as well or else you will not get good range or coverange, because Thread are low-power radios which commication uses short messages over 2.4GHz frequqncy range which means that have very poor wall penetration (similar to Zigbee).
Highly recommend following the same best practice tips for Thread as for Zigbeee to get the best possible experience → Zigbee network optimization: a how-to guide for avoiding radio frequency interference + adding Zigbee Router devices (repeaters/extenders) to get a stable Zigbee network mesh with best possible range and coverage by fully utilizing Zigbee mesh networking
When in doubt, just add more Thread router devices, i.e. the more mains-powered Thread devices the better, (at least until Home Assistant’s OpenThread Border Router add-on gets support for multiple Thread Border Router adapters as then another best practice will be to add more Thread Border Routers to the same Thread network).
I dropped in to Ikea Springvale (Melbourne, Australia) today, expecting to pick up some old zigbee devices, and found that they had a shelf with most of the matter stuff there - I wasn’t expecting it until Jan/Feb - it was definitely a work in progress, because while the products were there, there were no tags or pricing. I asked a staff member who looked it up for me. They had the temp/humidity, motion, door/window and air quality.
I hadn’t looked in to matter capability until I got home, so I was surprised to see that my google streamer showed automatically (which I expected), but my 4 google wifi devices also appeared on the thread tab.
I used the mobile app to share credentials for one, made it preferred, put batteries in the temp/humidity sensor and was immediately prompted to add it - added it, and it’s all worked fine straight away…
I’m still not clear on whether the device can now only communicate with the one “preferred” thread network, or all of my google devices, but will do a bit of reading/experimenting in the next few days. I got all the devices except for the air quality sensor. also, they didn’t have the water leak detector on the shelf yet.
@Gnnr yes so far, which is better than my expectations
@Hedda that is a good advise, my zigbee network is well mixed and covered, but I still have reservations about matter in general, and I don’t care for needing to use the phone to onboard (vs zb pairing which doesn’t). And like others, do have concern about @ Do Matter "owners" mind the integrated "kill switch"? 🧱 so I intend to invest slowly. That said this motion is in the same room as the thread dongle.
@farnsy I believe that matter devices can/should work across the fabrics, once you do the initial onboard with your preferred fabric/controller, you can then ‘share’ the device to other fabrics (in HA, share is on the device page).
I added this motion sensor to HA matter (instead of apple/google) for example, but alternatively, instead of sharing it, I used the @ GitHub - t0bst4r/home-assistant-matter-hub: Publish your Home-Assistant Instance using Matter. to export it, which makes selected entities automagically show up via other HA integrations like homekit/etc.
I actually just noticed that while the occupancy/illumance is showing up in google, they are not in homekit. The matter bridge is showing an error ERROR OccupancyS~ingServer OccupancySensingServer: Since revision 5 of the cluster features need to be set based on the detector type. Currently no features are enabled. that I will need to investigate.
Here in Spain are starting to be available in many IKEA stores. Like
I believe that parasoll are still the old zigbee devices, MYGGBETT appear to be the matter version of parasol.
this is what is listed on the US site @ MYGGBETT door/window sensor, smart - IKEA
