It's fine for answers to use them as support, but generally answers shouldn't rely on Crawford tweets alone
Jeremy Crawford explained the change in a tweet on January 30, 2019:
It looks like the Invoke Tweet of Jeremy Crawford feature has been nerfed. Official rulings are now found in the Compendium only, with this account merely providing a "preview" of possible future ones.
Yes, I decided I don't want people feeling they need to dig through my tweets for official answers.
Even if they aren't "official" rulings, Crawford's tweets still often suggest design intent and clarify ambiguities. They can provide useful support for a good answer to a question, as long as the answer doesn't rely solely on the tweet and nothing else. Often, Crawford's tweets merely reiterate what the rules already state, so the answer can elaborate on the tweets by directly citing the relevant rules and explaining how they provide the answer to the question.
Rulings are all interpretations of the rules. Whether or not Wizards of the Coast or Crawford himself considers a ruling "official", they can potentially be used to support a good answer to a rules question - but people shouldn't feel obligated to include them. If someone answers a rules question in a way that's contradicted by a Crawford tweet or even a Sage Advice Compendium answer, that's fine, but you should ideally address their existence and explain why you think they're wrong (or why you think they're not the best way to resolve the situation, or whatever). The best arguments for something often anticipate and address counterarguments.
What should we do about any answers that do rely on tweets alone?
This problem has pretty much the same solutions as with any other answer that's not as good as it could be: downvote it, post a comment suggesting how it can be improved, post a competing answer, and/or (as Medix2 suggested in a comment) put up a bounty to attract better answers.