no worries.
What drops always depends on the specific bossfight. The named bosses of the story have their own specific loottable. Darak #2 is e.g. dropping his Shield and Falstead.
The Pestilence Outbreak version of Darak has most likely another loottable and also drops CSB. At least that’s my experience.
You can remove all Pestilence Outbreak by running Crucible and buying the reset from Seneschal at floor five. I also bumped a Pestilence Zone once too, although not maxed, giving me a tough but nice Pestilence Boss encounter.
As for lifesteal, I don’t use it myself, but it’s quite a lot used by players reporting to have very powerful builds on that basis. Search this forum, there’s much wisdom here. Always drive your builds in a direction where your wanted feature is scaling and triggering other benefits, as discussed here: