I believe werewolves are unique enough to not be considered simple shapeshifters. Here are some common aspects werewolves have (although not all at once):
- They possess enhanced senses in human form.
- They can transform into a wolf-like form. The exact description usually varies but it's very often not a normal wolf, nor a human. A bipedal creature with wolf head and claws but otherwise human-like in shape is common.
- They are tied to the night and the moon. Often the full moon. The exact link tends to vary but often it's when the transformation can or does take place.
- There is an element of animalism and losing humanity to the wild animal in a lot of werewolf fiction. Werewolves in human form might prefer raw meat or trying to "hunt" (at least attempt to) for food. In their wolf-like form they might lose rational thought or find it very hard to think like a human.
- The werewolf condition is very often tied to "evil" in some way. A curse is a common way to become a werewolf, this curse can be passed onto others. A more "scientific" interpretation of the same concept is a disease. Historically, werewolves have been blamed on pacts with the devil or straight up evil spirits coming back to terrorise the living. Overall, it's not a condition one willingly takes and it's a burden.
- Silver is frequently a weakness for werewolves. Sometimes silver bullets, other times anything used to injure them that is silver, other times still the mere touch or presence of silver can affect them.
These are just some of the elements that werewolf fiction shares. Yes, there are many variants and it's hard to find any two that are exactly the same in the elements they cover and how they cover it. However, there are still certain set of expectations for a werewolf that don't hold up at all for shapeshifters.
A shapeshifter is anybody who can turn into other forms. There is a wide variety of them in fiction and myth but very often they:
- are willing - that's in contrast with a lot of werewolves where the transformation is forced upon them or becoming a werewolf is.
- turn into "normal" animals - a lot of shapeshifters can turn into a fox, or a dog, or a swan, or a rat, etc. Even a wolf. Some even are animals that turn into humans. However, the implication is that they are indistinguishable from other members of the same species. Not a human-wolf(/fox/dog/swan/rat/etc) hybrid monstrosity that nobody would consider to be natural.
Werewolves can broadly fall under "shapeshifters" because they change shapes but I'd argue that's not enough to merge the two tags. We haven't merged vampires with undead and I suspect most would agree we shouldn't. The main problem is that vampires are very specialised, not "just a sub-species of undead". Yes, you could argue that not all vampires are undead but that's a moot point - we wouldn't eliminate vampire just to replace it with disease or creature-design or biology or anything else (I hope) because vampire works for classifying questions correctly.
Above all, tags are for categorising the content. If a question truly does pertain to both shapeshifters and werewolves, it should be tagged as both. However, I'd have different expectation if it was tagged with just one or the other.