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In real life a pregnancy fails if a foals genetic make up is too broken. That can happen for all sorts of reasons but relevant to us are color genes that break important biological functions. For example frame brakes the ability to create lysine. The horse can get by with one working copy but if both copies are broken the foal can not survive.

Remember that wild horses are/were pangare bay dun and any variation of that is caused by a gene that broke. Some breaks like A becoming a are not a big deal but some like KIT becoming KIT w10 can be pretty powerful breaks that can mess up a lot on a cellular level. The more of these breaks you have the more problems you can get.

The ultimate defining of if a foal will survive after birth or if it will be even born at all is not an specific combination of genes. It is if the the unique combination of genes of that horse are such that things are so broken it is unsurvivable.

In the future some genes like frame will be strict two copies equals death, most genes will be any combination survives. Some gene combinations when combined with high white factors will cause lethal foals.

I can not give exact examples just yet because I don't know where all the chips will fall just yet, but just liker real life you will not be able to just look at two horses and know for sure if they will have a 25% chance of a lethal foal anymore. Some will have much lower instances of foal mortality then that.

So instead of w4 + w2 always being lethal they might only be lethal if the foal is w4w2 and extensive WF. (Example only, I don't know how this will end up for sure.)

No genes will gain lethality that were not lethal before, but many genes will be less lethal then before and have more crossing options. On those risky genes you will need to stick to low WF animals and limit other broken genes.

For example: a w4w2 might be fine but a w4w2 frame and splash 2 may be too broken to survive. Note that all of those genes are already genes that can be lethal in the right combos.