Many people are still not using gravatar or they do not want to use it and it would be effectively impossible to enforce a company wide policy on that.

In order to still be able to get the accessibility benefit of having avatars there is a small change that can be done for enabling auto-generated avatar icons for people that do not have a gravatar setup.

Read https://en.gravatar.com/site/implement/images/ and https://github.com/blog/1586-identicons

You will find that you can add an optional fallback image for the gravatar images.

<img src="http://www.gravatar.com/avatar/00000000000000000000000000000000?d=identicon" />
Or using github identicon:
<img src="http://www.gravatar.com/avatar/00000000000000000000000000000000?d=https%3A%2F%2Fidenticons.github.com%2F00000000000000000000000000000000.png" />

It would take 5 mins to implement this feature and it does not add any security risks if you re-use the gravatar ID.