With the War Within Pre-Patch, Shadowlands and Battle for Azeroth raid bosses are now harder.
We tested multiple raids and difficulties (including LFR) in the following expansions. We've discovered that all raid bosses have been buffed by the following multiplier when comparing their War Within Pre-Patch health/damage values to to their Dragonflight versions.
With Warbands being launched in the War Within Pre-Patch, many players have been looking to farm old raids for Transmog since it's much easier to do so. Unfortunately, this change makes legacy raids harder (or significantly harder in Shadowlands) and it may be worth bringing friend or friends if you're looking to complete the hardest difficulties/most recent raids.
Also remember that many of the BFA raid bosses have been made possible to solo now, but with these health increases it'll likely just be harder and take more time.