Technically, it is a setting in the UEFI (different technology replaced BIOS). That setting wasn't available in original SP4s and didn't come out until late 2018. MS probably added the capability after seeing the battery problems with multiple customers and having replaced many of them.
We were told about it way back in this thread. I had to update my UEFI to get the capability for my SP4, which was replaced by MS under the extended warranty in March 2019. I too have it plugged in most of the time. With the "Battery Limit" set in the UEFI, charge is limited to 50%. I expect it will last for years to come, maybe until MS stops supporting Windows 10 in 2025. That Battery Limit setting is available from the SP3 all the way up to SP8 (see this link).