User accounts do not have "expiration dates".

It's open source. You can code up any feature you want and submit it to the repo on github.

No idea. I've never given adding this feature any thought.

If it were me I would look at adding an entry to the ost_config table. (you probably want to automate this some how, maybe by writing a plugin or something)

Here's what the entry would look like.
id would be the next entry so you dont need to supply it.
namespace I would use user.# which would be username and the id of the user.
key would be something like expiration_date.
Value would be the datetime for when you want it to expire. so something like today at midnight 2020-03-26 23:59:00.

You would need to add a field to each user that you can define (datetime). And use this data to populate the ost_config table and code to display when you view a user.

then lastly you would want to figure out how you want to handle accounts that are past their expiration. Then write the routines / code to check that and handle what you want to have happen.