Just had a reply back from the XTEINK support who said:
"We can confirm that for all overseas versions of the X4, both those currently on sale and those already purchased, we have not restricted the use of third-party firmware.
Regarding any potential future adjustments to our machines, we are committed to listening to user feedback and opinions before making any decisions. Furthermore, we will ensure that any such changes are announced well in advance."
If they said this at the start, this whole thing would've been a non-issue. So it looks like we can relax for now, unless you've ordered Chinese stock from Aliexpress
EDIT: XTEINK have now updated their previously unclear statement that was made just prior to this post (now pinned to the sub reddit) to confirm: for all X3 and X4 Overseas Versions purchased through our official website, whether currently on sale, in transit, or already in users’ hands, we are not restricting the use of third-party firmware. We are not making changes that would affect how these existing devices are being used.