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The {INFO_HTML} variable is only available in the HTML Body of the mail template. 

Go to System, Mail Templates, Options, Mails and set Mail Format to Both. Click on Save & Close.

Then edit the “Admin Tools: Blocked request” mail template and you'll see two areas, Body and HTML Body. The Body cannot have {INFO_HTML}, the HTML Body can.

Yeah, I know, the interface is confusing. That's Joomla's mail template interface, not mine. It doesn't let me define different variables for plain text and HTML email, nor does it let me add a clarification about it either.

It depends. It does something in configuration monitoring, critical files monitoring, and super users monitoring. 

For the generic blocked request email it does not do anything. It hasn't done anything since many years ago. It is not even included in the default email template. It only exists so that old mail templates don't spit out the literal string {INFO_HTML}.

As for the accessed URL, the correct variable for that is {URL}.

Again, the INFO/INFO_HTML variables were only ever used way back in the day to report the username and password used when someone got blocked for failing the login. However, it turns out that many (most?) people don't use a password manager, therefore these emails ended up containing a slightly mistyped password by the same user over and over again which ultimately made it easy to determine what their password is, defeating the purpose. This was removed years ago, but the variable remained for backwards compatibility reasons.