Quick Carpet has a feature to spawn PlayerBots on the same postion where u killed them using /player xy login .

I am not able to find it in the FabricCarpet mod

This may have to do with the fact that fabric carpet is not quick carpet...
But here you can use keepalive.sc from the scarpet app store which will respawn fake players when you restart the server. Alternatively, you could modify it so it works on all players and add a command to do the same as /player xy login.

carpet shouldn't store any non-vanilla data if possible. Especially that there is an app that can take all the blame for extra non-vanilla data here.

As per Ghoulboy, you can achieve that with apps. Type /script download utilities/keepalive.sc and you should be good.