I'm not sure about user deletion logging, but if you uncheck the "really delete users" flag in Administration, Site Settings, then you won't accidentally lose users if you're concerned of that.

Some features in the system capture last updated datetime/last updated by information in the database, since mojoPortal 2.3.9.5. This was introduced along with the Recent Updates RSS feature. The HTML feature tracks that for sure.

The best thing to do is look at the mp_ tables in the database, if you have that capability, to see if the features you want to audit have those fields in place.

There is no comprehensive logging of things but there are a few config settings that you can put in user.config to log some things. These were mostly implemented to track when people mess up our demo site since we are letting strangers login as admin there and we don't want people to change the admin password or remove the instructions or product demos on our demo site. occasionally we get repeat offenders messing up our demo site and I have to ban their ip address so this logging was created to help with those problems.

at some point I may implement an add on feature for tracking user interactions with the site in more detail, but for now maybe these will help you some.

<add key="LogFailedLoginAttempts" value="true" />
<add key="LogIpAddressForContentDeletions" value="true" />
<add key="LogIpAddressForPasswordChanges" value="true" />
<add key="LogIpAddressForEmailChanges" value="true" />