In a strict sense, the patch does break backward compatibility for third-party IDLE extensions that modify the rmenu_specs contents. It is not a "private" value since it lacks an initial underscore in its name. But given how undocumented IDLE is, especially its extension facility, I don't think poses any problems.

(For what it's worth, I'm the developer of IdleX and I already released a trivial workaround for this change. I welcome this improvement to IDLE, especially when it deprecates an existing IdleX extension.)

While keeping UI consistency may be desirable, keeping it crippled is not. Even Window's Notepad has a decent right-click menu.