I’ve had multiple devices from years ago (alpha/beta test of Sync and Sync v2), I switched a couple of devices, added them to the sync chain; however, the older ones do not show the “x” button to delete them in my sync chain.
I thought that after a few months, the developers would resolve this, so I did not create any tickets/posts.
Using latest Brave (nightly) on Windows, and regular update channel on Android. on iOS was using regular channel as well (the iOS device is the one that cannot be removed).
Type of devices currently running on the Sync chain in question:
Android - 2 devices
Windows - 2 devices
Have you tried leaving the Sync chain on all your devices and just creating a new Sync chain? Is that a problem?
Below is some additional information in a similar topic which may be helpful in understanding the problem.
For whatever reason, if you are unable to leave the sync chain on the devices you still have and create a new sync chain, you should submit a request to support using the form linked below to see if support can manually remove the data. Make sure your include a screenshot of the brave://sync-internals/ page.