Increase available version in apt.

If you report a bug and the bug and security fixes are significant then the package will be updated sooner rather than later. Ubuntu is not a rolling release distribution and packages can be left unupdated in favour of one's with genuine issues. It is rare for Ubuntu to have the latest packages and stability is king. This is especially true in the LTS versions.

You may find a PPA here on Launchpad.net but these obviously come with the usual caveats of a PPA.
https://launchpad.net/ubuntu/+ppas?name_filter=ansible

Remember to filter the PPA for your release name. Not all PPAs support all releases of Ubuntu

Manfred Hampl suggests this article as an answer to your question:
FAQ #3037: “no rolling release”.

You have to be aware that Ubuntu is copying packages from Debian (to avoid double packaging work), and as far as http://packages.debian.org/src:ansible shows also Debian has only versions up to 2.9.9

First step towards ansible version 2.9.10 in Ubuntu must be creating a bug report in the Debian bug tracker, that they update their version. Ubuntu will then follow.

Thanks @manfred, I've logged the bug with Debian to update to 2.9.10.

@Andrew,
The latest release has a lot of bugfixes that may warrant an update in apt.

https://github.com/ansible/ansible/blob/stable-2.9/changelogs/CHANGELOG-v2.9.rst#bugfixes

That's great. I'm not a bug guy but if you report a bug then it may get the package updated sooner rather than later

Now that you have created the Debian bug https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=963482 also a bug report in the Ubuntu bug tracker should be created, with a reference to the Debian bug.

Remark:
There is a PPA available with version 2.9.10, https://launchpad.net/~ansible/+archive/ubuntu/ansible-2.9 that can serve as a workaround.