Many of the plugins do not have their availability displayed on Admin -> Plugins -> Plugins overview.

At a code level, the problem appears to be that class plugintype_qtype inherits the is_enabled implementation from plugintype_base, which only returns a null. Returning a null results in an empty availability table cell on the plugins overview page.

They fall into several categories:

1. Some plugin types cannot be enabled or disabled. For these, we should put a greyed-out message in the cell saying something like 'Cannot be disabled'. (Needs to be fixed in admin/renderer.php.)

2. Some plugin types can be enabled or disabled, but this functionality is not exposed on the overview page. (Needs to be fixed in lib/pluginlib.php.)

3. Some plugins may only be able to be enabled or disabled in some really complex way, so we can't make it available on this overview page. I'm not sure what the best fix is here. Not sure what the best fix is for this.

Oh, I see, the plugin overview page does not let you alter whether a plugin is available or not, it just displays the information, even though it uses something that looks exactly like the clickable icon for altering the availability that we use on pages like manage activity modules. That needs to be fixed.

For the plugin types that are currently missing, here is my analysis:

qtypes: 2
message outputs 2 or 3
filters: weirdly, this only shows disabled plugins! 3
editors: 1
course formats: 1
profile field types: 1
reports: 1
course reports: 1
grade import/export/reports: 1
adv grading: ?
webservice protocols: 2 or 3
qbehaviours: 2 or 3
qformats: 1
plagiarism plugins: ?
admin tools: 1
themes: 1
all the sub-plugin types: 1 - I assume?