Hello Mike,

The split in the lower trunk could be due to a range of stress factors or prior injury (and perhaps planting depth, as you mention), but it appears to be aged and well-"healed" in as much as plant tissues repair/protect themselves. Still, it is prudent to promote good air circulation around the scar by keeping it above-grade and free of mulch, as you have already done. No spray or other treatment is practical or even effective at protecting the wood, so no other direct actions are needed. Indirectly, keeping the plant as stress-free as possible will help keep it vigorous and robust and best able to fend-off infection/infestations. For yews, this essentially means keeping the soil free-draining and only watering them as needed during dry spells, since roots are prone to rot in consistently-wet conditions from compacted soils, poor drainage, or over-watering. Fortunately, yews are otherwise fairly adaptable plants, and can be very long-lived when growing well.

Miri