An AF SLR or DSLR have dedicated AF sensors on the floor of the camera under the mirror. In a mirrorless camera that system isn't there anymore. Instead AF is handled directly on the image sensor itself. Also the manuals for older speedlites refer to the red AF Assist Beam as IR (Infrared). But its not Infrared if it really was Infrared we couldn't see the AF Assist Beam. The camera relies on contrast and distinct lines for the camera to focus on. That's why the AF Assist Beam on older speedlites have distinct lines. Also a downside to the old school IR AF Assist Beam was when new AF systems came out with more AF points. Was that the grid pattern couldn't cover the new AF point layout. So if you chose an AF point not covered by the AF Assist Beam. The AF Assist Beam would NOT be projected at all. So in 2003 Canon dropped the dedicated body AF Assist lamps and started using Intermittent Flash Firing (*Disco Flash* as my collogue @kvbarkley calls it) from the built in flash. So this wouldn't be a problem anymore with the different AF point arrays. In 2009 Canon released the original speedlite 270EX it was the first speedlite in the EX range to lack an IR AF Assist Beam. Instead Canon chose to use Intermittent Flash Firing for the AF Assist Beam. The Intermittent Flash Firing AF Assist is COMPLETELY INCOMPATIBLE with ALL EOS AF SLRs. It is also COMPLETELY INCOMPATIBLE with EOS Digital SLRs released prior to 2007. In 2008 Canon added the External Speedlite Control to new cameras. The first speedlites to support this were the 270EX (original), 430EX II & 580EX II. Cameras released from 2007 onward have additional hardware to support Intermittent Flash Firing and an LED AF Assist Beam (320EX). All of Canon's speedlites have always supported Intermittent Flash Firing but the camera bodies lacked the necessary hardware to support it. The camera body rapidly varies the voltage going to the speedlite to control the brightness of the AF Assist pre-fire/ pre-flash. Newer speedlites they can adjust the IR AF Assist Beam they emit. To match the amount of AF points the camera has. The 430EX III-RT is only able to emit an IR AF Assist Beam to cover the center AF Point. By default the speedlite uses Intermittent Flash Firing to overcome this limitation. While the 600EX II-RT can cover 65 AF points. To cover older AF points it has different AF Assist Beam projectors to match the AF point arrays of older cameras. The speedlite's IR AF Assist Beam is linked to the camera's AF system to know which projectors to turn on. Below are some example pictures of limitations that the IR AF Assist Beam may suffer from on different cameras. Intermittent Flash Firing doesn't have the AF point restriction that IR has. This is due to Intermittent Flash Firing covering the entire frame and isn't linked to a specific AF point. Since the entire area will be illuminated when the AF Assist Beam is emitted by the flash.
Well the AF Assist Beam from the speedlite isn't actually IR. If it really was we couldn't see it then. But since we can see it is visible red light from the flash. Some old EZ series A-TTL speedlites used an IR pre-flash for the metering and it IS NOT VISIBLE at all. A-TTL speedlites fired the pre-flash with a half press of the shutter button and a sensor on the flash measured the pre-flash. Then set flash exposure accordingly and set the camera aperture. Note some EZ series speedlites fired a visible white pre-flash from the main flash tube when the speedlite was in the bounce position.
But newer cameras like the R6 mk II have *both* the pins for the MFA hot shoe and the traditional 5 pin connection for legacy flashes.
