They will also deregister you automatically after some period of time. What you described is the situation several years ago, but it's much better now.
That's a link to deregister a phone number from iMessage without an iPhone, which is good, but I don't see any text on that page that confirms that they'll deregister you automatically, or if there's any user-visible indication of the issue. If that's the case, then I'm glad they finally addressed it, because it was definitely a problem for far too long.
In that case, Signal's current behavior would be comparable to Apple's, if Apple also deregisters you after a period of inactivity.
In that case, Signal's current behavior would be comparable to Apple's, if Apple also deregisters you after a period of inactivity.