How to view systemd's runtime global config?

You are looking for systemctl show -all for the global property set or systemctl show $servicename --all for a particular service.

I found the answer by searching man systemctl for first "dump", and then "show", which found the command.

Or to answer your specific question:

$ systemctl show --property=DefaultTasksMax
DefaultTasksMax=18446744073709551615

DefaultTasksMax was added in systemd version 228.