Development versions of software
When version 1.98 was mentioned in the answer that is linked in your question, it said:
"Yes, this is in the latest development version (1.98) as I discussed on the Avogadro forum."
The key phrase here is "development version", which is usually a term that indicates that it's an "unreleased" version that is used by developers or other people who need to use some of the features that have not yet been "officially" released.
Software releases
Before seeing Geoff's comment here, I had already looked for version 1.98 at the Avogadro 2 GitHub page and the README file indicates that 1.97.0 is the latest "release".
When I click on the button that I circled in the above image, it takes me here, which indicates that 1.97.0 was indeed the last "released" version, and that it was released on 21 July 2022.
Building software from source
Since at the time of your previous question: How to address 'saveGraphic: Method not found' error in automating the generation of figures in avogadro using Python?, I knew nothing about the "nightly builds", my first instinct if I wanted a feature that is not yet in the latest "release", would be to simply build the code myself from source. Notice on GitHub that some of the project's files have been updated as recently as "yesterday", and other files have been modified "4 months ago", which both indicate that the version presented to you by default when you clone the GitHub repository is much more recent than 1.97.0 which was released in July 2022:
Fortunately the Avogadro 2 website has instructions about how to build the code from source.
This is the safest way to guarantee that you're running the most recent version of the code, because it will clone the current repository from GitHub, which may have have changes that are not yet incorporated into the "nightly build".
My first instinct when I saw your question was to try to build the code from source, because this is not only the safest way to guarantee that I have the latest version (nightly builds have been known to fail or not be up-to-date for some software packages), the safest way to ensure that the program will work on my platform (operating system version, etc.) and if I want to modify the source code (e.g. changing the units used for geometries), I would need to build from source anyway if I want my computer to understand the new code.
Nightly builds
A nightly build is essentially the result of a computer/server somewhere, "building from source" each night and offering you to download the result. When nightly builds are up-to-date, assuming that they were build correctly and from the most recent version of the software (it's not always obvious what the nightly build is doing, whereas if you build the software yourself, you have so much more control), they will provide you with a version of the software based on the latest changes to the code before the nightly build was performed.
Upon my first inspection of Avogadro's index.html page (the landing page on the website), this is possible for MacOS and Windows users:
When I visit install.html, I also see that an option to use a nightly build is available on Linux machines:
