Can we evaluate if these signatures can be dropped? Here's a few arguments:

• It can mask bugs when the first argument inadvertently evaluates to undefined, which results in all classes being removed the first time it is called.
• It's badly documented (Misleading documentation for toggleClass() method. #1696) and because of that, probably not well-known.
• The way it saves a __className__ data seems like a bad practice.
• For custom builds, it creates a otherwise unnecessary dependency for attributes/classes towards data.
• It would allow the following savings for full builds:

   raw     gz Compared to master @ 5b4cb0d33731a58384e02ad51e703e7dcb00e424
  -701   -230 dist/jquery.js
  -168    -61 dist/jquery.min.js