So I think I've found out a way!!!!!!!!
I was in the same boat as you, accidentally spammed a text (i didn't even know there is even an option to do so)
But yeah, accidentally spammed a text, freaking out, called apple, called my carrier, called back apple and no help whatsoever. So I started playing around with my phone a little more, here's the steps you can try to "unspam" the person you were texting that you didn't save the number.
1. Go to your messages.
2. Click on the "create new message" icon, the one on top right.
3. If you happen to know their first number that it starts with, type it in, ex. Number is area code 665. And I don't know the rest of it I can just type in 6. And a list would drop down.
4. Scroll down and you'll see the numbers on your contacts are highlighted in blue. The numbers that are not in your contacts that YOU PREVIOUSLY text would be in grey.
If you spammed or block whatever it may be, you can still click on that number (the grey one) and once you click on it there should be an option to unspam them. Or you can save the number to your contacts.
Hope this helps!!!
-cindy