up until now there actually hasn't been a domain name system TLD that's 100% safe to use on your own internal private network that you can't guarantee won't collide with some new TLD in the future if someone registers it
.local has existed for a while but it's used for link-local things, zeroconf and mdns, and you'll probably break something if you use it
but now we have .internal that you can use for your internal private networks without interfering with these services and without fear of future domain name collisions if it's registered