I have an early 2015 Macbook Pro running Big Sur. It's displaying a "Battery service recommended" notification since recently
I checked the system information. Cycle count is 437 and full charge capacity is 4770 mAh. That seems odd, since Apple estimates the warning to come up after some 1000 cycles.
I looked in preferences to see if I had checked "Optimised battery charging" but the option is missing (Preferences > Battery > Battery).
I wouldn't be bothered much since this macbook is not my daily workstation and I wouldn't mind even if it had to run on the power adapter all the time. But I'm reading about how MacOS reduces the maximum processor speed by half when the battery has this status. I've noticed how it's slower than it used to be (but I can't really tell how much of the difference is between my ears due to p[acebo/suggestion to be honest).
I can't find much information about it, but I'm interested to know whether the reduction of cpu power is just happening while on battery or also when on AC power.