Oooh this is interesting - I uninstalled Google Play from my phone a day or two ago, and now Signal is giving me a "permanent Signal communication failure" error message saying it can't function without it. I was able to send and receive a few messages this morning (it warned me when I installed it that in the absence of Google Play it may be "unreliable", but didn't say it would flat-out refuse to operate), but I guess now I can't use it at all.
I just re-installed Signal this morning after a forced week-long lockout that wiped all my message history. Only a couple people I message with are willing to use it instead of SMS anyway, so I guess it doesn't matter much, but yeesh Google really is the central pivot of a LOT of stuff.