Blade Runner is somewhat notorious for being a film with multiple versions, which largely came about as a result of studio interference, time constraints and rights issues (see https://en.wikipedia.org/wiki/Versions_of_Blade_Runner for elaborate information). This raises the question to which specific version Blade Runner 2049 is a sequel.
Director Denis Villeneuve has stated that for him, the most important versions of Blade Runner are the original Theatrical Cut, and Ridley Scott's Final Cut, and even though both he, Scott and Harrison Ford have their own ideas about which version is canon, the plan was to make a sequel that could be a follow-up to both these versions. In both versions, Deckard displays the same red glare in his eyes that established Replicants have: a subtle hint that he could be one as well, but it is never stated that this is a unique Replicant trait. The Final Cut makes a stronger case: there is an added scene of Deckard daydreaming about a unicorn; at the end, Gaff leaves a origami unicorn for him as he allows him to escape with Rachael. Many people interpret this as a hint that Gaff knows what Deckard dreams about, so the memory must be an implant, and Deckard must be a Replicant.
In Blade Runner 2049, Gaff is asked if he knew that Deckard would run and go into hiding. He confirms, saying that he saw it in his eyes. This can be interpreted in the figurative way, meaning that Gaff could 'read' Deckard's sentiments and guessed that he wanted to flee with Rachael to save her. It can also be taken literally: Replicants can be identified through the V-K test via their pupillary reflex, and they display a red glare in their eyes. So Gaff may have realized Deckard was a Replicant, and gave him a hint about it (through the origami unicorn) so that he would run away, for his own safety and Rachael's. Gaff also makes an origami sheep, which possibly teases the title of the novel that Blade Runner was based on (Do Androids Dream of Electric Sheep?). But in either case, there is no definite confirmation that Deckard is human or Replicant.
Other 'hints' are also non-conclusive. In this movie and its predecessor, Deckard seems physically inferior to Replicants, but this is no definite confirmation of his humanity; he could simply be a next-generation Replicant without a termination date, normal strength and implanted memories (like Rachael was described as 'an experiment'). While in Niander Wallace's office, it is suggested that he and Rachael were set up by Tyrell to meet each other and reproduce. This would seem to be easier if both were Replicants and thus Tyrell's products (they could have been designed to be attracted to one another). On the other hand, in such a tightly controlled world, Tyrell may have had plenty of knowledge about human Deckard, his strengths and his preferences. Rachael may have been designed according to Deckard's likes (she does show some resemblance to the pictures of women in Deckard's apartment), and Deckard could have been specifically selected to come to Tyrell and meet Rachael (which would explain why he was forced to do it, even though he was officially retired). The movie also leaves open the question whether Rachael only lives 4 years (as suggested by the Final Cut) or has an open-ended lifespan (as clearly stated in the Theatrical Cut): it is explained that Rachael died during childbirth, almost two years after fleeing with Deckard, but before her potential expiry date. So the film evades several of the issues brought up by the differences in version of its predecessor, allowing the viewers to decide for themselves which backstory they want to consider.