The mechanic for taming boar, wolves, and lox in Valheim is a two-step process. First, one must confine them in some sort of enclosure from which they cannot escape. Then one must provide food and remain in the area while they eat it. These animals are tamed by associating being fed with the presence of a viking.

One does not have to chase feral boar, wolves, or lox into a pen. Since all three animals will attack a viking, it is simple matter of getting them to chase the viking into the pen.

Deer, on the other hand, will invariably flee from a detected viking. They will not chase a viking, one cannot lead them into a pen. Deer must be herded into a pen. Since deer can outrun a viking, this does not work.

If a deer cannit be penned and can outrun a viking there is no way to feed a deer and then maintain the proximity necessary to tame it.

Perhaps Iron Gate Studio will decide to change deer AI to allow herding them. Until is does so, attempting to tame deer is a waste of time.