The terse form of the advisory states:
> To help keep your account secure, starting May 30, 2022, Google will no longer support the use of third-party apps or devices which ask you to sign in to your Google Account using only your username and password.
It's the innuendo that's interesting. The message in the subtext of this statement is, Look at these apps! They want you to use them for e.g. checking your email, but look at what they do! Isn't it awful? In order to let you check your email, they make you give them the password for your _whole_ Google account!
Of course, the only one who's responsible for the current arrangement is Google. Google, not third-party developers, are to blame (and _solely_ to blame) for why access to the various Google services is consolidated into a single account. Google, not the Thunderbird team, are to blame for why your Gmail password is the same as your Google Vault password, which is the same as your YouTube password, which is the same as the password you use to mark your phone as needing to be locked out of your account after it's stolen.
* This is why I'm skeptical of the whole "writing forces you to be honest because it means you have to actually think things through well enough to put them into words that can be put into coherent sentences" meme. Nobody seems to talk about how writing and the revision process that's inherent to it also provides the opportunity to finesse words. Some idea can be made to appear as if it's sound and backed by solid reasoning even when the truth is actually much less straightforward—or even contradictory.