Well, our Jenkins jobs still work decade after. Jenkins itself requires some care and feeding tho, mostly upgrades, especially if you use some plugins.

Then again jenkins job can be as simple as "download a repo and run some code in it" without anything fancy so even if SHTF you can migrate away relatively painlessly

No, they are all horrible monstrosities. The ones that are hosted by others (github actions) are still monstrosities, only that other people are taming the monsters.

The best way I've found is to keep the CI/CD tools as just simple task runners. They should only have to figure out when to run things, inject the necessary environment(and secrets), and figure out dependencies (if applicable) and run those. Whatever they run, you should be able to run the same scripts from your own laptop or a similar.

Alternatively

https://docs.dagger.io/1200/local-dev/ https://docs.drone.io/cli/drone-exec/ https://github.com/nektos/act

Jenkins/Travis are getting to this point, not that they are the "cutting edge" or anything. I'd personally rely on projects like woodpecker/drone/dagger so that there's no real "rely" on anything closed source or proprietary.