This may also be the case for URA (unregrettable attribution?). Someone I know left just short of two years for personal reasons, was never put in PIP/Focus, and then they tried joining a new team after and was told when they left their manager put them as URA, which prevents them from coming back. I've heard it can just be a year, though.
Have you ever dated someone that you can’t quite break up with for whatever reason, and weren’t bad necessarily, but still breath a sigh of relief that it’s over when they dump you?
The work equivalent to that is a URA, for a manager.
If you’re that manager and now ‘single’ (have open headcount) and looking to find someone to ‘date’, would you hire that person back, or go find someone else - even if they were a complete question mark?
If hiring managers/recruiters have more candidates than they can handle, a prior URA can impact a candidate the same way. ‘There are many fish in the sea’
Does that mean you’d never get hired back? Nah, it happens. But it isn’t likely to help.
Unregretted attrition, which means you no longer work there and you're not sorry about it.
Which means you are the one with the power in the negotiation (the party with the most power is the one who needs the other the least).
So they have a codified sour grapes rule to punish people who they don't have power over? To punish people who weren't begging to please continue being allowed to work there? That is actual insanity right there.