Even if you’re still running macOS High Sierrra (I’m on 10.13.6) you’ll receive a Safari update (from 12 to 13). This will break a number of extensions – including most adblocking – also a vim-based keyboard extension I was using called Vimmy.
So you may want to avoid upgrading, or do as I’ve done and switch your default browser to Firefox etc. for the time being.
- 32-bit apps will no longer be supported at all (currently that still includes Acquia DevDesktop – for running Drupal development sites)
- the default shell is going to change from Bash to Zsh
- scripting languages won’t be installed by default (Apple gave Python, Ruby and Perl as examples in a support document, but didn’t specifically mention PHP). It’ll mean Homebrew will need to be installed differently. Presumably it won’t be an issue if you’re upgrading a previous macOS version, but, also presumably, it will be if you’re using a new computer, even with Migration Assistant.
(As usual, my own policy is not to upgrade to a new macOS release until 9-12 months after it’s come out, to allow everything to settle – I write this on a machine that’s still running High Sierra.)