I have mixed feelings on all of this, but I agree that it really ought to not do that by default.
The ProTip[tm] I read sometime back was to:
1. Buy a prepaid SIM.
2. Put it in your phone.
3. Activate Signal with it.
4. Remove the SIM and put it in an envelope somewhere in case you need it again.
5. Put your real SIM
And now, Signal will work but not be tied to your actual phone number.
So there's a workaround, anyway. (Assuming a subsequent Signal update hasn't broken this.)
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