I am not talking about the PIN you would have to enter when starting Signal, to get into Signal.
I Googled a bit and found an approachable blog post from the time this all happened, here;
https://blog.cryptographyengineering.com/2020/07/10/a-few-th...
This has refreshed my memory of events.
In short, Signal wanted to store what had been purely client-side information (contact lists, for example) on their server, but - in principle at least - in a form Signal could not access.
The PIN in question is used to provide access to that information.
> Server state comment aside, it seems your main complaint is about a pop-up PIN entry UI that can be opted out of?
The dialog to force the user to set the server-side PIN disabled the app. You either had to do it, or stop using Signal. There was no opt-out.
I had a look at the app now. I found the settings you mentioned. It's not clear to me from what I see there is this if an app-locking PIN, a SIM protection PIN, or a server-side state PIN, or all three rolled into one.
In any event, at the time it happened, the presented dialog was full-screen and could not be dimissed; even if there had been options to disable this (and there were not prior to the full-screen dialog - I looked, in an effort to dismiss the permanent partial-screen dialog) you could not get to them, because it was a full-screen dialog which you could not dismiss; you could not get to the app, and so could not get to settings.
The only option was to stop using Signal or provide a PIN so your client-side state could be stored server-side.