That's interesting - I too had vouchers expiring on 30 December, but have heard nothing about an extension. Given that they sent me details of an extension from their original expiry (ie the expiry date when I got them for a trip I didn't make after the pandemic hit in Spring last year) well ahead of that expiry date, I assumed that if there was to be a further extension then they would again have said so in good time without being asked. So in the absence of any such notification I decided I'd have to use the vouchers - but at least I can, under current ticket conditions, subsequently change the dates again and again if I end up having to further delay using the credit. So I don't feel I'm really any worse off.
One problem, however, was that the tickets I wanted to use my credit for were London to any-Belgian-station in June. But the any-Belgian-station tickets were only bookable up to April, even though Brussels trains were bookable well past June. Why would that be? Even if Belgian timetables aren't fixed that far ahead, the any-stations ticket doesn't relate to any particular Belgian onward service (ie you don't even need to specify your destination, let alone a specific train - you just use it for whatever train you want after you get to Brussels), so I can't see the logic in not selling any-station tickets as far ahead as Brussels tickets. I've sent two messages to Eurostar about this lack of advance tickets, but so far had no response.
I thought that a way round it would be to book the any-station ticket for a time when they were available (ie in April), and then reschedule with a free ticket exchange when the later dates became bookable. But the cheapest tier of tickets available in April is much more expensive than the ones available in June, so that would have meant spending a lot of money on top of the vouchers; whereas losing the flat-rate add-on in June is likely to cost me only a bit, much less than getting higher-price tickets in the first place.
