Why don't Intel microcode updates work on my system?

Based on results of /usr/sbin/iucode_tool -tb -lS /lib/firmware/intel-ucode/* no microcode is being loaded for your cpu because at the moment there is none. That doesn't mean that there won't be in the future. You can safely leave intel-microcode and iucode-tool installed, if there is an update containing microcode for your cpu's signature it will then be used.

Bug in Meltdown/Spectre Intel Microcode

There was a bug in early 2018 Intel Microcode update to address Meltdown/Spectre security holes. As such the microcode had to be rolled back to a previous version.

Here is the microcode I'm using (having opted out of all updates starting January 2018):

$ apt list --installed | grep intel-microcode

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

intel-microcode/now 3.20170707.1~ubuntu16.04.0 amd64 [installed,upgradable to: 3.20180108.0+really20170707ubuntu16.04.1]

When you install Intel Microcode Update you will get this version or something similar:

intel-microcode/3.20180108.0+really20170707ubuntu16.04.1

Ubuntu 16.04 LTS Menu

I'm not sure about Lubuntu menu structure but for regular Ubuntu this is how you access the Intel Microcode Update controls:

The bottom option controls Intel Microcode Updates.

Install Intel Microcode from CLI

To skip the GUI menus altogether you can install from command line:

sudo apt update
sudo apt install intel-microcode

dmesg now shows correct output

After following the installation steps dmesg returns the desired output (unlike in your question where it shows nothing):

$ dmesg | grep -i microcode
[    1.166542] microcode: sig=0x506e3, pf=0x20, revision=0xba
[    1.166993] microcode: Microcode Update Driver: v2.2.
[16082.584598] microcode: microcode updated early to revision 0xba, date = 2017-04-09

Ubuntu had released fixed kernal update against this exploit. The new kernal have required changes.

Reference: SecurityTeam/KnowledgeBase/SpectreAndMeltdown | Ubuntu Wiki

I am in Ubuntu 16.04, Using Nouveau display driver. In my case, Intel-microcode got uninstalled after installing new kernel.

This update is released right after intel had released bug fix for spectre bugs security vulnerability.

This should solve your problem.

If you got intel property driver replaced like in my place and still want intel-microcode property driver.

Intel Releases Linux CPU Microcodes To fix Meltdown & Spectre Bugs | bleepingcomputer.com

Currently, the new drivers are not included in Ubuntu ppa. Users may have to download it manually from Intel website.

Warning: This may cause driver conflict or instability in your Ubuntu. You have to install it on your own risk.

Download Linux Processor Microcode Data File | downloadcenter.intel.com

For me, I have no issue in my ubuntu laptop. So I don't want to take risk. I left Ubuntu team to decide what is best from my system. I am waiting update from Ubuntu driver team.