Fixing deprecations is unfortunately the lowest prio of any kind of work for majority of the projects. Part of the problem is probably lack of pressure to do so it if the timeline is unclear. What if this is actually never removed? Why going through the pain?
IMO telling "we deprecate now and let's see when we remove it" is counterproductive.
A better way: deprecate now and tell "in 12 (or 24?) months this WILL be removed".
After 12/24 months, cut a new semver-major release. People notice the semver-major through the dependency management tools at some point, an maybe they have a look at changelog.
If they don't, at some point they may want to use a new feature, and finally be incentivised to update.
If there's no incentive other than "do the right thing", it never gets done.
Having said that, I think LLMs are really going to help with chores like this, if e.g. deprecations and migration steps are well documented.
Alternative option: create a codemod CLI that fixes deprecations for the users, doing the right thing automatically. If migration is painless and quick, it's more likely people will do it.