This was mentioned in the forums some considerable time ago, this recent post only confirms this. But a firm date hasn't yet been decided and or given. However, isn't likely too far in the future.
I have many programs in daily use that are aside from no longer being developed are highly unlikely to run on win10.
As for the problem you mention about "18.4 that affects many XP installations (failure of UI to load)" I haven't experienced that problem on my XP or Win10 systems. I have retained an older version of avast on my win7 netbook, Under Powered (Intel Atom N255 1.5GHz dual core CPU), damn small screen (and resolution) and only 2GB of ram.