They Exist in the final days

First of all, there is no rule that bans their existence, and with Two Tails there is at least one canonical example. But there are more, usually among the Archetypesand each Tribebook delivered on one "pregen archetype character" with a history of its own, but which can be seen as a canonical character too.

• The Ice Dancer Archetype was born from a Vietnamese-Canadian Stargazer and a French-Canadian Silverfang.¹
• The Class Clown Archetype visited a Kinfolk Academy in a class of three without knowing their parents.²

They didn't always exist

The tribebooks make clear, that the stance on Crinos-born shifted, and preventing actively or culling them has not happened for the last 200 years. They are described as... well, regrettable it seems in the first Tribebook:

Garou shall not mate with Garou

We've obeyed this tenet scrupulously throughout most of our history; metis Silver Fangs were all but unknown before the 18th century. Since that time, however, worsening situations have led some of us to turn to our own kind for comfort and procreation. While these characters are in the vast minority, their number is growing. Offspring of these unions are pariahs in most houses, and are particu- larly unbalanced.³

They are the lowest members of the Tribe

Now, the best description of how the Silver Fangs handle such is from the other Tribebook:

Garou Shall Not Mate With Garou

All Garou know the punishment for breaking this tenet: the birth of a metis Garou. These poor creatures bring shame on us all, for they are living reminders that we cannot control our passions in the name of Gaia. Those who commit such sin should be singled out and marked for it: the sigil of the oathbreaker should be carved into their backs, where they cannot erase it. As for the metis, they are not to blame, but neither should they be held as anything but impure. They should be given a place in Garou society, but among the lowest.⁴

It is clear from both Tribebooks that the attitude of "smother them in the crib" has been abandoned long ago. It's the parents fault, the deformed child can still be an asset in Garou Society, even if as a bad example and at the outskirts. The Class Clown again tells us that the tribe sequesters their crinos-born and teaches them in one location as smaller groups.²

1 - Tribebook Silver Fangs, 1st Edition, p. 58.
2 - Tribebook Silver Fangs, Revised Edition, p. 90.
3 - Tribebook Silver Fangs, 1st Edition, p. 30.
4 - Tribebook Silver Fangs, Revised Edition, p. 52.