To prevent impersonation, Twitter has switched from brand profile avatars to squares.

The company’s perennially puzzling authentication makeover has taken a new turn, and brand profiles are now square to better display them in-stream.
The fundamental problem with Twitter’s new $8 verification scheme is that it quickly opened up a new channel for impersonation in the app, causing a slew of issues for a variety of companies within hours of the option’s availability.

Twitter has since added gold checkmarks for brands (which were also recently updated) so that users know who the official brand account is, and these new square tiles add another level of assurance, as people will eventually come to understand that real, official brand profiles appear in the app as squares, not circles.
As a result, it provides an additional layer of security against impersonation. Which makes sense, but only in the context of the first verification redesign, which makes no sense in and of itself and was a muddled, misguided, and ill-informed effort to begin with.

However, given that this is the direction Twitter has chosen, increased protection against impersonation can only be a positive thing. But you might have to adjust your company avatar for the new format.