So this whole "upgrade after probation" horse has been beaten to death, but people are still relying on bad info soooo ...
The unfilled vacancies exist so that the company can assign them to new hires. Currently those are all NBFO spots. There was a short time period in late '23 -- April '24 where they had unfilled 737CA vacancies, and those were assigned to new hires. For those that took those slots, they had to complete 500 hrs at UAL and get off probation (a year) and then they'd be eligible to go to upgrade class. Many of those folks are in upgrade training right now.
However, that ship has sailed. The last class to have NHCA slots was April 2, 2024. (I missed this by one class). Since then, our regular vacancy bids have seen NBCA slots go much more senior. I think the most recent bids had the most junior 737CA at an 11,000 employee number (out of 18,000 total), and we just had a bid with the most junior Bus CA at around 13,000 (too lazy to look it up).
The monthly crew resource updates clearly state that we are more than fully staffed for NBCA positions for the foreseeable future. We have a lot of brand new CAs in training right now and hitting the line, so if you were to start indoc now, it's anyone's guess how many years it would be before you see the left seat. Same goes for WBFO, that has gone super super senior over the last year I've been here....