I am on v2023.1.12.
The context menu to the hierarchical thesaurus in the keyword panel no longer provides for renaming, deleting, adding hierarchical keywords, available in v2021. Is it correct that these functions must now be done through the thesaurus manager (ctrl+T)?
Yes.
See #01852 in the IMatch 2023 release notes (https://www.photools.com/release-notes/?productId=505).