Generally delusions are suspected when a belief conflicts with someone's (sub)culture or deeply held beliefs. If a belief is found within someone's culture or subculture it doesn't typically qualify as a delusion, because it is then understandable and may serve other functions. Physical therianthropy (the act of physically shapeshifting into another animal) has existed as a belief in all sorts of cultures, and the Western shifter community is one form of this. So that reduces the possibility that shifters are all delusional - if we were this belief would not have persisted for so long throughout the ages.

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Delusions can be neutral or rarely positive, but generally they tend to impact people (or occasionally others) negatively, hence why they're listed in the DSM. Unlike religious, spiritual or supernatural beliefs they are not adopted and practiced; you cannot decide to become delusional or learn how to be delusional like how some people claim to have learned shifting. If a delusion is positive and doesn't appear to be affecting one negatively in other ways, psychiatrists may leave it alone, treating it similarly to other religious beliefs.

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The ICD-11 says this about the boundary with normality:

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A continuum of delusional beliefs, attenuated delusional beliefs, overvalued ideas, and unusual or eccentric beliefs has been observed in the general population. Such beliefs may be more common among people under conditions of adversity. People with Delusional Disorder may display greater psychological distress, greater preoccupation, and higher degree of conviction compared to people in the general population with beliefs that are similar in nature to beliefs that could be characterized as delusional.

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So generally, unless you're distressed, highly preoccupied (to the point of it affecting your life negatively) and unreasonably closed off to other possible explanations, your beliefs are not indicative of an illness. Many people have some kind of odd, unusual, or borderline delusional beliefs, but they don't usually become a cause for concern until they affect someone's life negatively.

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A delusional syndrome that people often try to link to shifting is clinical lycanthropy. However, in both clinical and modern cases I've seen it is typically a distressing and dysfunctional syndrome to live with. Clinical lycanthropes don't generally enjoy shifting or being an animal the same way shifters do, and often experience other symptoms that signal a psychiatric or physical disorder, such as inability to control shifting and other negative supernatural phenomena, distress or inability to function, or disorganized thoughts.

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I hope this reassures anybody who was worried that shifting might be delusional for them due to the stigma surrounding it. Other people may freak out about physical shifting because it's not the norm, but it isn't an inherently delusional or pathological condition or belief. If it isn't causing issues in your day-to-day life and you're not distressed by it, you're fine. (And if you are distressed/having issues, you should seek help. You deserve to feel comfortable in your body!)