Battery disconnect is intended for disconnection to allow safe servicing of the laptop. Your best bet for extending battery service life is according to some to limit the charge maximum which is available in the BIOS. Personally I would not bother. I have a Thinkpad T480s with the original battery with no charge limits used on it in the last three years, that spent a lot of time plugged in and it still charges to 89% of the designed maximum.
As to a battery switch to turn it off internally between mobile and desktop…lots of issues spring to mind with that issue. Your battery is going to lose power over time anyway even if it is unplugged you switch it back to mobile … oh crap I am at 80% charge but I have to leave NOW … yeah screwed. I have it on desktop, power fluxuation, or split second outage, my laptop crashes, the cheap nvme drive I went with does not have power loss features and now my data is corrupted and the damn thing wont boot… FRAMEWORK WHY WOULD YOU SELL ME A LAPTOP THAT THIS COULD HAPPEN ON!!! … extra screwed, yeah I think everyone will want to pass on this idea.
Anyway I think this is a problem that is not a real problem with modern hardware and that the implementation of an option would simply create real issues that result in data loss. The ROI in this situation is negative. I would rather have the battery service life end a coupe of months sooner and replace it when needed than create an option where data loss is very real for the average user who does not have a real backup plan.