There are arguments for and against disabling low data rates but disabling lower legacy data rates will help your WLAN perform much better.
In 2.4GHz, the goal is to get rid of 11b client devices. Use of 11b clients necessitates use of low (non-OFDM) data rates forces the use and ripple of protection mechanisms (e.g. RTS/CTS and CTS-to-Self) which can impact the overall performance of your WLAN.
Using “No Legacy” will cause 11b and 11g devices not to join the SSID and will prevent protection ripple, both of which will add capacity to your network.
The PHY header is always sent at the minimum supported PHY rate for the band, it’s important to remember that the clients have to decode the MAC header in order to understand the frame type, and that includes beacons and probe responses. The MAC header is part of the PSDU/MPDU and is sent at higher/configurable rates.
“High Density” not only disallow 11b and 11g clients from trying to join the network, but only allow clients will a good signal level join, which could impact your roaming behavior.
In environments with older scanner or POS devices proceed with caution.
Most 5G capable client drivers have been written to accept any rate or set of data rates that an AP is announcing as Basic. Further still, there are a few 5G capable client drivers out there that will only accept 6, 12, or 24M, but nothing else. This is why 18M, when used as the only basic rate, may not be a good idea – as it will cause clients to not connect to the BSS.
There is great material available on the internet on this topic.