A User in our organisation came across this issue today whereupon they needed to use the "Harvard" Citation style in the document they were working on, but were only able in MS Word 365 to choose the "MLA" or APA" options:

There are various Archived Help Posts where others have reported this issue, all now locked so I cannot add my feedback to them, so, I've started this new Post to record my findings.
The above referenced posts mostly refer to try flipping the Office Language to English (US), or to restore missing Stylesheets to the Bibliography/Styles directory (under Program files or Appdata, or both).
None of this worked for me, the Stylesheets weren't missing, and flipping the language had no impact. Based on what I later found, it can't have made any difference anyway.
They key thing noted was that of the many stylesheets present in the folder structure, only two were picked up by word. With a little CoPilot assistance in comparing the files, a difference stood out - the Ones that worked were the Only Two that had an XML Parameter called:
OfficeStyleKey
We can easily see which Stylesheets have this defined using the following query;
Get-ChildItem "$env:APPDATA\Microsoft\Bibliography\Style\*.xsl" |
ForEach-Object {
$hasKey = Select-String -Path $_.FullName -Pattern "OfficeStyleKey" -Quiet
"{0,-40} {1}" -f $_.Name, $hasKey
}
With Result:
APASixthEditionOfficeOnline.xsl True
CHICAGO.XSL False
GB.XSL False
GostName.XSL False
GostTitle.XSL False
HarvardAnglia2008OfficeOnline.xsl False
IEEE2006OfficeOnline.xsl False
ISO690.XSL False
ISO690Nmerical.XSL False
MLASeventhEditionOfficeOnline.xsl True
SIST02.XSL False
TURABIAN.XSL False
Only the APA and MLA Stylesheets have it present.
Then, experimentally, We restored this relevant parameter back into the HarvardAnglia Stylesheet;
<xsl:when test="b:OfficeStyleKey">
<xsl:text>HARVARD</xsl:text>
</xsl:when>
This needs to be added to the Version for the user's AppData folder to be effective, though it would be wise to add this into the Program Files version (C:\Program Files\Microsoft Office\root\Office16\Bibliography\Style) as well, as new profiles would be generated from this.
Once this is changed, and Office is restarted, the Harvard Option becomes available in the Word UI Drop down picker;

And you could repeat this for each of the Referencing Styles that you wish to restore.
Hopefully in a Future Update, Microsoft address this by updating the Stylesheets deployed with Office 365.
Until then, I hope this tip helps others who have encountered the same issue.