A complete Greenland slide-off would be an average sea level rise of about 7m, and is possible in our lifetimes as an extreme event (something like a fraction of a percent chance before 2100). If it happened it would be multiple events really, spread out across years or decades. Antarctic ice moving so its weight is no longer supported by the continent was too unlikely to include in models a few years ago, but the West Antarctic has been so active that I’d expect it to start showing up in estimates.