I want to uninstall Eclipse version Mars 2, but it doesn't appear in the "Programs and Features" area of the Windows Control Panel. The installer has installed start menu shortcuts and other artifacts, so it's not just a matter of deleting the installation folder.
There is no automated uninstaller.
You have to remove Eclipse manually by deleting some directories and files.
Note: I use Unix style paths in this answer but the locations should be the same on Windows or Unix systems, so ~ refers to the user home directory even on Windows.
According to this discussion about uninstalling Eclipse, the reasoning for not providing an uninstaller is that the Eclipse installer is supposed to just automate a few tasks that in the past had to be done manually (like downloading and extracting Eclipse and adding shortcuts), so they also can be undone manually. On Windows there is no entry in "Programs and Features" because the installer does not register installations in the system registry.
The default installation path is in ~/eclipse/.
The Eclipse installer also keeps track of installations. Launch the installer and then open the "Bundle Pool" dialog (in simple mode it's in the menu - in advanced mode it's a small button next to the bundle pool drop down) and click on "Analyze Agent". You'll see list of profiles. Double click on a profile to see the path of the installation.
You could also search for files that contain "eclipse" in their name with any file search tool to locate installation folders. But be aware that Eclipse is a platform that other software can build upon, so there might be folders of Eclipse-based applications that look very much like Eclipse IDE installations but are actually software you might want to keep.
If you have installed Eclipse manually without the Eclipse installer, just delete the Eclipse directory and be done with it.
If you have installed Eclipse with the Eclipse installer, then delete the Eclipse directory and any desktop and start menu shortcuts. Then launch the Eclipse installer and open the "Bundle Pool" dialog and click on "Cleanup Agent" to remove unused bundles. This won't affect other Eclipse installations if you have any.
In my opinion this is generally enough and I would stop here
If you really want to remove Eclipse without leaving any traces, you have to manually delete
~/eclipse/photon/)The installer has a "Bundle Pools" menu entry which lists the locations of all bundle pools. If you have other Eclipse installations on your system you can use the "Cleanup Agent" to remove unused bundles. If you don't have any other Eclipse installations you can delete the whole bundle pool directory instead (by default ~/p2/).
If you want to completely remove the Eclipse installer too, delete the installer's executable and the ~/.eclipse/ directory.
Depending on what kind of work you did with Eclipse, there can be more directories that you may want to delete. If you used Maven, then ~/.m2/ contains the Maven cache and settings (shared with Maven CLI and other IDEs). If you develop Eclipse plugins, then there might be JUnit workspaces from test runs, next to you Eclipse workspace. Likewise other build tools and development environments used in Eclipse could have created similar directories.
If you want to delete your projects and workspace metadata, you have to delete your workspace(s). The default workspace location is ´~/workspace/´. You can also search for the .metadata directory to get all Eclipse workspaces on your machine.
If you are working with Git projects, these are generally not saved in the workspace but in the ~/git/ directory.
Even though Eclipse isn't "installed" like a regular Windows application, there may still be a few entries related to Eclipse in system registry.
If you search the Registry for "eclipse" you may find some entries for URI handlers, that are used for e.g. letting you install Eclipse plugins from a link on a website. Entries for the handlers eclipse+command and eclipse+mpc are created if the user selects them in the Eclipse settings in the "link handler" section. An entry for the eclipse+installer handler is created option to install Eclipse from links is activated for the Eclipse installer. In both cases this is an something the user has to activate. Eclipse does not write these entries automatically.
On my system I see a few other entries which I believe are created by Windows itself and not by Eclipse (e.g. entries in the list of recently used executables, file type associations, settings for how to run the Eclipse executable etc.). Whether you want to delete those entries is up to you. Personally I wouldn't, but on the other hand it probably won't break anything if you do. Just be aware that registry "cleaning" is a controversial topic.
In some cases other applications write entries about Eclipse. For example Eclipse asks you to exclude it from security scans by Windows Defender on first start up. If you accept this, Windows Defender will create entries that likely can only be removed by Windows Defender.