@King_1988 check these cheat sheets from WLANPros (https://wlanprofessionals.com/revolution-wifi-mcs-to-snr-levels/) and (https://wlanprofessionals.com/mcs-table-and-how-to-use-it/).
Depending on the manufacturing process, there is a minimum SNR from where the receivers cannot distinguish between the valid signal and the background noise, so they cannot decode it to get the data been transmited. One of the advances of new technology, apart from providing new modulation techniques, new standards and new features, is to develop new radio chips with increased sensibility and capabilities to decode weaker signals. But not all vendors use the same quality for all their chips and this is hand-by-hand with the price of the harware.
At the end it's all about testing and stablishing a limit to the minimum SNR your devices can work with some reliability and not start retransmitting a lot, and another limit for the performance you and your users want for applications, which is not the same limit for collaboration applications such as Teams or Zoom, than for web browsing or emailing.
That 20-25dB limit is set for minimum modulation schemes to match Wi-Fi 4 data rates, but using more conservative modulations we can set the limit down to 11-15dB, but at the cost of performance and beyond that at the cost of packet loss and disconnections.
