Lots of replies here pointing out that that any kind of "provisional employment" wouldn't work for the majority of candidates who are currently working.

From the candidate's POV what is mostly broken about the application process is all of the ATS gaming, resume tweaking, etc, just to avoid getting filtered out before an interview, as well as the inane leetcode screening that some companies are doing.

A better process might be to replace all initial profile/resume-based screening with task-based evaluation (evaluation could be at least semi-automated - if a computer/AI is going to reject me, I'd prefer it to be based on task evaluation job skills rather than ATS-filter avoidance skills!).

Lengthier on-site interviews/evaluations could also be task-based - for a developer role perhaps a 2-4hr peer-programming or problem solving task. Far less signal that a 3 month provisional hire of course, but maybe a better use of everyone's time than a traditional talk and brief whiteboard challenge process which is clearly failing as a useful filter.

I wouldn't expect companies to forgo the traditional touchy-feely team/culture fit type of screening as well, but better if this came after they'd already determined, as best they can, whether you've got the chops to actually do the job well.