Depreciated by 'who'?
It's a standard, not a feature from some company that is ending support. There is a governing body.
"The Khronos Group, Inc. is an open, non-profit, member-driven consortium of 170 organizations developing, publishing and maintaining royalty-free interoperability standards for 3D graphics, virtual reality, augmented reality, parallel computation, vision acceleration and machine learning.[1][2] "
OpenGL will continue to be supported for the foreseeable future but it will not gain any major new features, and it will likely lose direct support (requiring emulation or shims to actually work) on many platforms that still do support it now within a decade. For many games, this actually isn't a big issue, as they're not pushing the envelope anyway. But for those that do (or potentially will), OpenGL is the wrong choice.
By the very Khronos Group you mentioned. They are developing Vulkan now, which they consider the successor to OpenGL. There will be no new OpenGL versions, again it is deprecated technology. Deprecated by its creators just like Python 2.
And just like Python 2, you can still use it, but again it would be a questionable choice for a new project in both cases, and for the same reason.
It isn't like because Python 2 was depreciated, you have to switch to C#. It isn't gone. You can use the new version. Or older version until ready to migrate. Maybe it seems like it because of a name change, and you don't want to call Vulkan, OpenGL
So why give up on OpenGL altogether, when there is continuing development.
Going from Python 2 to 3 wasn't a cake walk. Hence so many years to migrate.
But I get what you are saying. Some upgrade paths are easier/harder than others.
Yes, even despite of many similarities and shared stuff, which can't really be said about OpenGL and Vulkan.
I think the term "deprecate" implies some level of actively discouraging use that is simply not happening with OpenGL (and that was/is for sure happening with Python 2). Specific platform vendors may deprecate OpenGL on their platform (as notably Apple has done), but that does not mean OpenGL is deprecated by Khronos Group.