When I was browsing options, Tildes didn't even allow sign-ups. I tried multiple days and eventually said fuck it and checked other options, settling on kbin.
Tildes was always invite only. Granted it is (or was at the time, maybe they slowed down with the influx) really easy to get your hands on one by just asking on r/tildes (or email).
Hell, the site being readable by the general public is a relatively recent introduction compared to it's history.