Is "this'll" considered proper English?
It is a perfectly normal contraction, in daily use by millions of English speakers.
It is also widely used in informal writing. Many people regard contractions of this sort as inappropriate for more formal writing.
Whether you regard it as "a word" or not is up to you.
"This'll" is an example of a word that is spoken aloud frequently but is not part of standard written English. Even if one is writing with other contractions (can't, won't, I'll, etc.), "this'll" would not be written down and would instead be expanded as "this will."