I am running CentOS 7.3 on my server. CentOS 7.3 comes with a fully patched version of PHP 5.4.16, with patches for later versions backported by Red Hat.

Unfortunately, the creators of Joomla! do not seem to know this, and deemed all PHP 5.4.16 installations a security risk. Therefore, when Joomla! is upgraded to version 3.7, a red error bar is prominently displayed in its control panel:

Nothing wrong with letting the administrator know that the PHP installation may be end-of-life (and PHP 5.4 won't be supported by Joomla! 4.0 anyway). But there is everything wrong with not being able to disable this nasty "Error" popup (even though Joomla! runs just fine with PHP 5.4.16).

After wading through a lot of arrogant, snotty discussions in Joomla! support forums, I finally found a solution. Apparently, this error message is presented by a plugin that, like all plugins, can be disabled:

Cool! Just what I was looking for. Good bye, nasty error message.

Mind you, it is probably a good idea to re-enable this plugin before any major upgrade, just in case it has something more important and relevant to say.