What is the significance of dumping the tablespaces?
Tablespaces are used to define the physical file location of one (or more) tables - chances are if you were using them for something beyond the default innodb space, you'd know about it. It's likely safe to ignore.
The error will have started when MySQL was updated to 5.7.something, they introduced a breaking change in a minor version because it addressed a security flaw, and world + dog was greeted with this error message.
There's a way around it:
drush sql:dump --extra-dump=--no-tablespaces