civix upgrade + provide proxy access to snippets.json
civix upgrade is just that; will help keep things up to date, make things work better with smarty3+, php8.x etc. It does not increment the civi version requirements.
The other change is that snippets.json was not loading on one of my servers. This is because it was configured not to serve .json files (it's fairly rare to have to do so). This file is now accessed via a URL at /civicrm/themetest/asset?need=snippets instead. This should work across all CMSes and across all hosting setups.