For example, I can change my username here on this instance. So long as my email id remains the same, my account is operable.
But GitHub doesn’t allow me to change my username. On top of that, GitHub forces an identifier on you when you create your username.
My comment was mostly hoping only to make a generalised point; although open source software often has certain (good) features, simply being open source doesn’t mandate or guarantee any feature at all.
On the username subject in particular then absolutely; an inability to change your username is not the same level of ‘bad’ as things like lock-in or spying. As other commentors have mentioned I’m sure it’s not intentional at all, and simply a consequence of the implementation difficulty with federating that sort of change.
Like the same way you can’t ‘change’ your email address. The email address is your identity - if you want a different one, you need to make a new one.