Some of the recent changes to this module were perhaps not handled as
sensibly as they could've been. The broad strokes seem sane enough, but
a few key aspects affecting the user experience were left too rough
around the edges for my taste. My main gripe involves the partial
deprecation of the `nicknames' entry for the option `erc-button-alist'.
After some reflection, I'm now of the opinion that imposing a specific
constraint on a specific slot in a specific default entry that may not
even be present after customization is just too messy a notion to foist
on users or to try and communicate in a doc string.

A better approach, I now feel, is to remove the troublesome `nicknames'
entry altogether and take special care behind the scenes to gracefully
accommodate users who still have it present in their configs. The
attached changes attempt something like this and also tidy up other odds
and ends in erc-button.el. Note that the second patch also appears in
bug#63569 and is only included here for its ERC-NEWS changes, which
touch on some of the above. Thanks.