So yes, I use backports. Not marking this as "solved" because package is broken. Synaptic marked Lazarus 3.6 as an "available upgrade", I clicked OK and instead it installed v3.6 and kept v3.0, so effectively I had two versions installed. And when I uninstalled both, and tried to reinstall Lazarus, it installed v2.2.
Under preferences I have "install latest version".

# apt install  -t daedalus-backports  lazarus-ide 

should have done the trick.

You are mixing stable and testing. I would call this idea at least challenging, its no good idea if you do not know exactly what you are doing.

The metapackage lazarus-2.2 re. lazarus-3.0 should install the complete suite.

To get hold of version 3.6 you must have added an external repo. What exactly did you do regarding this "trick" to synaptic?

there is also a Qt5 version which may explain why your "apt-get install lazarus-ide-3.6" didn't quite get the job done.

thanks for keeping us posted!

apt remove --purge --autoremove lazarus-ide
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following packages will be REMOVED:
  lazarus-ide* lazarus-ide-3.6* lazarus-ide-gtk2-3.6* lazarus-src-3.6*
  lcl-3.6* lcl-gtk2-3.6* lcl-nogui-3.6* lcl-units-3.6* lcl-utils-3.6*
0 upgraded, 0 newly installed, 9 to remove and 0 not upgraded.
After this operation, 710 MB disk space will be freed.
Do you want to continue? [Y/n] y

apt-get install lazarus-ide
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following additional packages will be installed:
  lazarus-ide-2.2 lazarus-ide-gtk2-2.2 lazarus-src-2.2 lcl-2.2 lcl-gtk2-2.2
  lcl-nogui-2.2 lcl-units-2.2 lcl-utils-2.2
The following NEW packages will be installed:
  lazarus-ide lazarus-ide-2.2 lazarus-ide-gtk2-2.2 lazarus-src-2.2 lcl-2.2
  lcl-gtk2-2.2 lcl-nogui-2.2 lcl-units-2.2 lcl-utils-2.2
0 upgraded, 9 newly installed, 0 to remove and 0 not upgraded.
Need to get 10.6 MB/83.7 MB of archives.
After this operation, 666 MB of additional disk space will be used.
Do you want to continue? [Y/n] n       
Abort.

I have synaptic configured to install the latest version. Yet it chose an old, obsolete version of Lazarus.
I had to force it to install v 3.6

apt-get install lazarus-ide-3.6
Which did not install the main files. Just non functional icons and menus.

apt-get install lazarus-ide-gtk2-3.6
The following additional packages will be installed:
  lazarus-src-3.6 lcl-3.6 lcl-gtk2-3.6 lcl-nogui-3.6 lcl-units-3.6
  lcl-utils-3.6
The following NEW packages will be installed:
  lazarus-ide-gtk2-3.6 lazarus-src-3.6 lcl-3.6 lcl-gtk2-3.6 lcl-nogui-3.6
  lcl-units-3.6 lcl-utils-3.6

Which fixed the problem...
Don't know if I should mark it "solved". Anyone attempting to upgrade Lazarus to the latest version will have the same problem.
Anyways, thanks for the help.
I was uninstalling/re-installing it through synaptic, not the command line. Which gave me the error in the first message.

https://www.baeldung.com/linux/remove-package-completely

The problem is I keep getting nagged to remove Lazarus:

apt-get upgrade
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
The following packages were automatically installed and are no longer required:
  lazarus-ide lazarus-ide-3.6 lazarus-ide-gtk2-3.6 lazarus-src-3.6 lcl-3.6
  lcl-gtk2-3.6 lcl-nogui-3.6 lcl-units-3.6 lcl-utils-3.6
Use 'apt autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

"no longer required"? What's this , AI deciding what I do or don't require? LOL
Seriously, how do I get rid of that message? I've tried uninstalling and re-installing, message is the same.
PS I don't want to lose Lazarus configuration files.
TIA