Will the wheel bearing and hub from a 99 outback work in a 1990 legacy?

The only difference between the two is all '99 had abs and only some of the '90s did, so you might see a toner ring on the '99 hub that will be useless on your '90 if it did not have abs. But it is the same hub and bearing and seals, so you can still use it. There will just be an unused toner ring, but it is out of the way and will not bother if unused. The knuckle is the same also, except the hole to accept the abs sensor is smaller on one than on the other. There again, if your '90 does not have abs, no worries. If it does, you can modify the hole to accept the '90 abs sensor and still use it.