From the tidyverse design guide
When using ... to create a data structure, or when passing ... to a user-supplied function, add a . prefix to all named arguments. This reduces (but does not eliminate) the chances of matching an argument at the wrong level.
E.g. in nest() the ... are used to create a data structure that's why it uses .names_sep instead of names_sep.
In the past unnest() used ... to create data structures and therefore the old arguments are prefixed with a .. But now ... is deprecated and you should use the cols argument. Therefore, the new argument are not prefixed.