Sure, you can do whatever you want. You could even use non-rfc1918 addresses and nobody can stop you. It’s just not always a great idea for your own network’s functionality and security. You can use an unregistered TLD if you want, but it’s worth knowing that when people and companies did that in the past, and the TLD was later registered, things didn’t turn out well for them. You wouldn’t expect .foo to be a TLD, right? And it wasn’t, until it was.
Sure, you can do whatever you want. You could even use non-rfc1918 addresses and nobody can stop you. It’s just not always a great idea for your own network’s functionality and security. You can use an unregistered TLD if you want, but it’s worth knowing that when people and companies did that in the past, and the TLD was later registered, things didn’t turn out well for them. You wouldn’t expect .foo to be a TLD, right? And it wasn’t, until it was.