What changes for you? Nothing unless you start using threads. You probably weren't using threads anyway because there is little to no point in python to using them. Most python code bases completely ignore the threading module and instead use non blocking IO, async, or similar things. The GIL thing only kicks in if you actually use threads.
If you don't use threads, removing the GIL changes nothing. There's no code that will break. All those C libraries that aren't thread safe are still single threaded, etc. Only if you now start using threads do you need to pay attention.
There's some threaded python code of course that people may have written in python somewhat naively in the hope that it would make things faster that is constantly hitting the GIL and is effectively single threaded. That code now might run a little faster. And probably with more bugs because naive threaded code tends to have those.
But a simple solution to address your fears: simply don't use threads. You'll be fine.
Or learn how to use threads. Because now you finally can and it isn't that hard if you have the right abstractions. I'm sure those will follow in future releases. Structured concurrency is probably high on the agenda of some people in the community.
Im not worried about new code. Im worried about stuff written 15 years ago by a monkey who had no idea how threads work and just read something on stack overflow that said to use threading. This code will likely break when run post-GIL. I suspect there is actually quite a bit of it.
Please don't - it isn't relevant.
15 years ago, new Python code was still dominantly for 2.x. Even code written back then with an eye towards 3.x compatibility (or, more realistically, lazily run through `2to3` or `six`) will have quite little chance of running acceptably on 3.14 regardless. There have been considerable removals from the standard library, `async` is no longer a valid identifier name (you laugh, but that broke Tensorflow once). The attitude taken towards """strings""" in a lot of 2.x code results in constructs that can be automatically made into valid syntax that appears to preserve the original intent, but which are not at all automatically fixed.
Also, the modern expectation is of a lock-step release cadence. CPython only supports up to the last 5 versions, released annually; and whenever anyone publishes a new version of a package, generally they'll see no point in supporting unsupported Python versions. Nor is anyone who released a package in the 3.8 era going to patch it if it breaks in 3.14 - because support for 3.14 was never advertised anyway. In fact, in most cases, support for 3.9 wasn't originally advertised, and you can't update the metadata for an existing package upload (you have to make a new one, even if it's just a "post-release") even if you test it and it does work.
Practically speaking, pure-Python packages usually do work in the next version, and in the next several versions, perhaps beyond the support window. But you can really never predict what's going to break. You can only offer a new version when you find out that it's going to break - and a lot of developers are going to just roll that fix into the feature development they were doing anyway, because life's too short to backport everything for everyone. (If there's no longer active development and only maintenance, well, good luck to everyone involved.)
If 5 years isn't long enough for your purposes, practically speaking you need to maintain an environment with an outdated interpreter, and find a third party (RedHat seems to be a popular choice here) to maintain it.