Hi,
in the old version until nextCloud 13 you could set the expiration date for a file sharing per link to max. 30 days. The user could leave this at the date or choose a date between Now and 30 days.
Can this be configured again in the nextCloud 15?
The administrator can set a maximum expiration date under Settings > Administration > Sharing and a user can still adjust this date in the sharing dialog 
of a file or directory.
I have enabled the following option in the nexcloud(15): āAllow users to share via linkā, āEnforce password protectionā, āset default expiration dateā, Expire alfter 30 days and āEnfoce expiration dateā.
If the option āEnfoce expiration dateā is enabled, than user can no longer edit the expiration date.
If the option āEnfoce expiration dateā is deactivated, than user can freely select the expiration date or remove the expiration date completely.
Youāre right. I checked it with my admin account which allows to overrule a set date. But as a normal user it is indeed not possible to set a different date below the defined threshold. If this is an essential feature for you, you might open a feature request in https://github.com/nextcloud/server/issues .