Answering my own question over 8 years later:
The Bludit Content Management System is a free and open source flat file content management system that satisfies all the listed requirements (see below).
Bludit includes all the basic features you would expect in a content management system such as pages and posts, categories, tags, comments, WYSIWYG editor, hit counter, navigation, social networking links, search and sitemap.
More advanced features such as contact forms, galleries, popups, basic e-commerce, WordPress importer, RSS feed, embedding a Google Map and embedding a YouTube video are available via the plugin repository which currently lists over 100 plugins. Most plugins are free. The cost of the paid plugins is very reasonable.
✅ Free and Open Source
✅ Responsive web design as per https://en.wikipedia.org/wiki/Responsive_web_design The default install of Bludit includes responsive themes and there are plenty of others to choose from on the theme repository.
✅ Separation of content from design (e.g. via templates or themes) - Bludit uses themes.
✅ Extensibility - Bludit uses plugins.
✅ Ability to run on industry standard shared hosting (e.g. cPanel
etc.) Bludit is a flat file PHP CMS so the hosting requirements are minimal.
✅ Simple for non-technical users (e.g. website maintainers should not
have to learn Markdown etc. to be able to update content) - Bludit includes the TinyMCE WYSIWYG editor plugin as part of the default installation. If you are using a Bootstrap theme, the excellent "Bootstrap Toolbar" plugin can be added to the TinyMCE WYSIWYG editor to enable easy column layouts in content.
✅ SEO friendly (SEF URLs, ability to enter page titles and meta
descriptions etc.) - Bludit includes SEF URLs. Unique page titles and meta descriptions are generated automatically. An Open Graph plugin is included as part of the default installation.
✅ Simple updates so that maintaining the CMS is easy - Bludit updates, theme updates and plugin updates can be done by unzipping the install files over the current files. Plugins are also available to enable automatic updates of Bludit and all plugins and themes from the repositories.
✅ HTML5
✅ W3C valid - (Assuming you choose a suitable template.)
✅ Fast - Bludit is fast and scores well in test tools such as GTMetrix.
✅ Sitemap (suitable for submission to Google Webmaster Tools) should
be automatically updated as content is added / updated - Bludit includes a Sitemap plugin as part of the default installation.
✅ Support forums for the CMS - Support forums are available in English, German and Spanish.
✅ An established community rather than relying on one or two people to
keep the CMS alive - Bludit currently has 75 contributors on Github.
✅ A growing interest rather than a CMS in decline - The Bludit usage statistics and market share trend over the last 12 months looks very promising.
✅ A small footprint - A default install of Bludit uses 7.1 MB of disk space which is small compared to Joomla (the current latest version 4.2.5 uses 87.2 MB) or WordPress (the current latest version 6.1.1 uses 66.5 MB).