Is thank you one word or two. But the shift from two word to one word forms is a well established process in the language. You know that they are two separate wordsiif you use the full sentence which is hidden underneath i thank you from this comes the shortened version which we hear daily thank you. My nephews teacher recently wrote thank you as one word and i nearly had a fit. Whether thank you or thankyou is the correct spelling for the noun is a matter of opinion and language development.
But remember just as important is knowing that its always two words when you say thank you to thank someone. See this discussion for an explanation of correct usage. Unlike thank you which is still quite transparently two separate words i thank a verb and you the object of the verb goodbye is by now an unanalysable unit that cannot be split up unless you prefer to write it etymologically as good b ye. Alas it is two words as a not so kind english prof.
Texting was not a word until recently either. Welcome to the decade of texting and destroying the english language. This is the verb to thank with a direct object you. Gonna spell it as two words here but is the word thank you one word or two.
In fact you will know that this is two separate words if you use the full sentence which is hidden underneath i thank you. Thank is the verb and you is the subject. It is always two words. I would seriously advise against doing that.
There are many common english words that started out as two word phrases eg. But just like a lot people are now writing it as one. I usually spell it as two words but thank you for asking it prompted me to check and i found spelling it as one word is not always incorrect. Somebody everyone today or tomorrow.
Over time they became fused into the one word forms we use now.