They made an announcement about maintaining two branches: one based on OpenWRT 21.02 and another on 23.05. 4.5.x releases will be based on the older version of OpenWRT. 4.23.x releases will use 23.05.
I’ll give 4.5.8 a try once it’s released but only if it supports MU-MIMO. Can you test that version with your FRITZ!Repeater? For now, I’m back on 4.5.4.