I don't know. Depends on the firm, and also the practice within the firm.Interested how firms get rid of associates to realise this - does it tend to be through formal redundancy or 'managing out'?
They probably don't want to straight up fire anyone, since that's bad PR (and it's very difficult to fire without cause). More likely is that people just naturally leave and the numbers more or less work out. If they ever need to top up on associates they can always lateral new people it.
E.g. at Slaughters they will tell you by about 3-4 PQE whether you're on partnership track. If you're not, it's not like you will be fired. You'll just stay at associate/managing associate/counsel level and never get promoted. Most people take the hint and leave (many of whom end up as partners at other firms), but, even if they stay, given their experience they're still going to be billing lots for the firm so it's a win-win from Slaughters' perspective.
But there's always edge cases. E.g. this article (former Latham HK counsel sues the firm in part because the partners kept lying to her about her chances of making partner)