I don't think it's compatible with the sprit of FOSS.
You're giving the company (in this case Signal) an open-ended license for your contributions whilst they don't return the favour. And so whilst they can offer a proprietary version e.g. Signal Pro with a friendlier license e.g. BSD you can't.
And what kind of underpins all of this is that many companies especially in the enterprise will not touch anything that is GPL or AGPL.
You're giving the company (in this case Signal) an open-ended license for your contributions whilst they don't return the favour. And so whilst they can offer a proprietary version e.g. Signal Pro with a friendlier license e.g. BSD you can't.
And what kind of underpins all of this is that many companies especially in the enterprise will not touch anything that is GPL or AGPL.