math : botrk buff is a thornmail nerf.

in average bruiser does 300basic damage lategame +120 botrk damage (3k hp target)

before botrk buff : If target have in average 250armor, it mean you will take back 300x0.3=90damage from basic dmg but you will hit for 85.71dmg with basic attack and 34.28 with botrk. and lifesteal 8.57hp/hit. so actually you take back only 81.43dmg (90-8.57) and does 120dmg. thornmail return in this case 67.85% of dmg (lot less if you have mr, armor pen etc)

with the botrk buff (patch 5.8). you still does 120dmg. but lifesteal 3.428hp/more so you only take back 78dmg instead of 81.43dmg. So thornmail only return 65.0%

basically 65/67.85 show that thornmail got nerfed by 4.2%. and instead of returning "30%", it now return only "28.74%".

Thornmail user will be sad.