That’s not really a big deal since the session encryption was insecure anyway. It feels almost like a honeypot after they've removed forward secrecy. If you’re looking for a decentralized alternative SimpleX Chat is a more secure option.
Considering how fiercely anti-encryption the UK is/has become (because "only child molesters care about encryption!"), this is sadly reason enough for me not to trust it.
Do I believe they have a backdoor in their software? No.
But if the UK passes a law demanding they introduce one...
What government are you certain will never introduce a backdoor requirement?