I thought signal switched to allow usernames a few months ago

I wouldn't recommend that, since it's likely that anyone who could register with Signal using the same phone number could harm your ability to communicate with other people reliably, and I don't have many reasons to maintain a VoIP telephone number: https://www.privacyguides.org/articles/2022/11/10/signal-number-registration-update/

If someone gets a new phone number from their carrier, they should not be prevented from registering with Signal indefinitely because the previous owner has reglock.

The intention of reglock is to prevent hijacking of numbers you actually own, not to guarantee the number for yourself for life.

While this change makes sense from the perspective of making it so you cannot "hold a number hostage" as long as you keep checking in, it is particularly important for people who've used disposable phone numbers to know this.

I found that URL from https://www.privacyguides.org/articles/2022/07/07/signal-configuration-and-hardening/ which I found from https://www.privacyguides.org/en/real-time-communication/

There is some documentation that supports this idea, like https://support.signal.org/hc/en-us/articles/360007059792-Signal-PIN

Enabling a registration lock triggers a 7-day inactivity timer if your number is registered on another device.

Registration Lock expires after 7 days of inactivity.

After 7 days of inactivity, a new PIN can be created. The old PIN and information associated with it are no longer available.

I also recall that Mental Outlaw discussed this in the past, but I have failed to find where.