Signal, somewhat famously, when subpoenaed to hand over data, can only hand over the date that the account was created, and the last time it was used.

That's old news really. Signal is subject to the laws of whatever country they operate in. As such, a country can compel them to start logging information, and they must comply.

This is what happened with ProtonMail with a protestor in France/Switzerland. I forget which way round it was, I think they were protesting in France but went back to Switzerland, then because of an agreement between France and Switzerland and because it was a crime in both countries the Swizz government was able to compel ProtonMail to start tracking their IP addresses and used this to locate and arrest the protestor.

They most likely can't compel Signal to give them the messaging data. If they did, this would require a new key exchange, which they could MITM, but this should issue a notification to both users. It's possible a government could compel them to do this silently, however this would require app updates and I'd hope this would be where Signal draw the line. However ultimately there's no way of telling if the US haven't already done this.

Open Source ftw. Matrix is far better than Signal in this regard, in fact I rate Threema over Signal, although it's only slightly better.