I had a similar problem (albeit 3 years later) and after some research I had a process for potentially identifying the source of the problem and subsequently found that my issue was hidden in a plugin (iThemes Security in may case).
The missing Appearance > Editor in WordPress could be due to a number of potential causes. A process to identify the source of the problem is as follows:
- Systematically switch off plugins and check if the editor returns (use ctrl F5 to refresh each time between checks)
- If the problem is not found in the Plugins, swich all plugins back on, then temporarily switch the theme to a default WordPress theme (usually Twenty Sixteen as this is the fall-back WordPress theme) refresh and check.
- If editor is still not visible switch back to the original theme, then go to Dashboard > Updates and reinstall your WordPress version and check again.
It is likely that the problem would have identified itself or resolved during these checks and updates.
In my case, having identified the problem area as iThemes Security, I did a further search and found that there is an easy configuration fix in iThemes Security
Dashboard -> Security -> WordPress Tweaks -> Configure Settings
Uncheck: Disable File Editor