Most sensible Otherkin believe that you can only astrally shift, as in shift the way your spirit appears on the astral planes. Every time physical shifting is brought up anywhere in any context it creates a whirlwind of pointless site drama where everyone loudly exclaims their views and no one is convinced, so I'll avoid that. Phantom limbs are commonly mentioned as being experienced by Otherkin, but that is not the only cause necessarily, so I'd suggest you research it.
On your profile you say you will speak up if you see anyone being rude, yet here you are, being rude. Otherkin/Therian are believed in by plenty of people who are sensible, skilled, rational casters. The ancient legends from our world have many stories of humans who were not completely human, and many witches regularly interact with the Gods and Spirits from those tales. Is the belief in Otherkin really such a stretch, when the same tales that support our magic speak of beings like them?
Yes, I have met cringey Otherkins. I have also met cringey Wiccans, cringey Heathens, cringey traditional witches, and cringey people of every walk of life. But often it is the person, not the belief. Do the Wiccans who shove religions together without any thought mean the ones who are not are stupid? There are racist heathens, does this mean that the many who are not are cringey as well? There are many paths in magic, and if you want to mention science disproving Otherkin, science pretty much believes no form of witchcraft exists.
If you want to disagree with Otherkin? Fine. Go for it. Everyone has a right to disagree with whatever they want to. But disagreements of any kind should be respectful, and at least discussed, not just name calling.