I’m running V.24.8.2.1 on Windows 10 Pro 64bit. I installed the Oxford English Spellchecker extension and it knocked out Synonyms, greyed out Thesaurus, and removed English Sentence Checker from the Tools menu. When I removed it, Writer returned to normal. This Issue needs to be looked into.
You will need to log in to the extensions page and contact the author. Thank you.