Signal, like most services, block text verification services, free texting apps, etc.
e-SIM wise, depending on where you are, that might require identifying yourself, and depending on your threat model, having to purchase one in person or with payment info that can be traced back to you might be too risky. Same thing when it comes to using one in a device you own, or in a location that can help de-anonymize you.
In the end, Signal does this because they know the ban hammer would come down hard on them from the Justice Department and every state AG and legislature if Signal allowed bad actors to anonymously use their app and network to commit crimes.
The issue is that there are plenty of people who are not doing heinous things whose security and anonymity might be at risk because of the measure put in place to placate governments.
