Updating LanguageTool
Make sure to read the Gotcha at the end of the post before starting.
LanguageTool has recently been updated from v6.5 to v6.6. If originally installed using homebrew then most things should work when you run brew upgrade. However, I found I needed to restart it a couple of times until it would work. In addition, the upgrade creates a new plist file which restricts access to languagetool-server to localhost only.
So, curl -d "language=en-US" -d "text=did the upgrade work" http://localhost:8081/v2/check gives the expected half page of JSON verbiage, but there's no access to the server from remote machines on the network (or from elsewhere if you have it on a public-facing machine). This includes from browsers with the languagetool plugin installed, or Clemens Ertle's invaluable Obsidian community plugin.
The solution is simple, add <string>--public</string> to /opt/homebrew/Cellar/languagetool/6.6 as described previously.
Homebrew also now appears to list fasttext as deprecated or disabled (depending on where you look; brew info fasttext says it's deprecated, but the webpage states it's disabled) so we can probably assume it may not be installable via homebrew in the future 😞. However, you can compile and install fasttext from source easily.
Gotcha
Finally, a related #gotcha … updating LanguageTool overwrites the custom dictionary. Make sure you backup a copy of /opt/homebrew/opt/languagetool/libexec/org/languagetool/resource/en/hunspell/spelling_custom.txt before you update things.
Don't ask me how I found the last bit out 😞.