I literally just answered this question yesterday, as did my fellow moderator and author of the Intel temperature guide Computronix.
Hello! I managed to get my 9700K to 5.1 GHZ Stable on 1.39 Volts average. Maximum peaks are up to 1.44V - those are only peaks, it doesn't stay there. Temperatures are good -> 68-74C when playing CPU intensive games. Is it safe in the long term? Should I keep it like that? Thanks!
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I would not recommend using over 1.4v for any 8th or 9th gen Intel processor, although Silicon lottery says they are safe up to 1.45v. Likely, that is more appropriate for CPUs that have been delidded, since that is primarily what Silicon Lottery does is sell delidded CPUs, golden samples and offer delid services. 1.43v is not terribly extreme, by comparison, but if you expect this CPU to last three years or more I would not use more than 1.4v.
I see a lot of people getting stable results on the 9700k at lower voltages than you are using, by a fair measure, so it should be possible although you WILL want to do the testing to verify stability at whatever you settle on.