Yes it entirely depends on whether they store previously used usernames along with the date range it was in use (to tell apart multiple people who used the same username at different times)
We’ll have to see if any unsealed cases in the future support that they don’t keep those records like how they don’t keep IP logs, but personally their track record is enough for me to have confidence in the feature, especially since my “threat model” is primarily opportunistic hackers or spearphishers at most, not police or state / nation state level actors.
The idea is that you change or remove your username after someone else starts a conversation with you, so the username can no longer be used to subpoena your account details.
Put another way, signal is able to provide those 2 pieces of information to law enforcement based on a phone number. This helps you to prevent law enforcement having a phone number to ask signal to look up in the first place, assuming you change your username every time you hand it out.
They also hash the usernames that they store on your account which means law enforcement can’t ask what usernames are being used, only being able to ask for specific usernames which are currently in use.
I actually just realized lineage 21 does this too - I didn’t notice because cropping a screenshot with Google photos seems to remove all the fields (I also have the build string and timezone offset). Which is weird because cropping an actual photo the same way - as you would expect - preserves all the notable fields like timestamps, phone model / lens info, and the same “Software” field which for my photos is just “HDR+ 1.0.commithashlookingstring”