To add onto this, the problem with dumphones that can use Signal is that they’re often just dumb Android phones. That means they’re still connecting to Google Play Services. The main benefit would be in that the phone is so limited that you end up reducing the overall amount of data you put through the device, but it can be as privacy invasive as a regular smartphone.
Here’s another thread where we dug into this with a specific dumbphone example.
All depends on your threat model, so this is all just a bi FYI so you know what’s the deal.