Because of how audio works, to slow down the song without also lowering its pitch, osu! needs to resample the audio file. However, the process for this isn't perfect, so there will be tiny "mistakes" in the slowed-down version of the song; these mistakes are called artifacts.
Because of these resampling artifacts, individual sounds in the slowed-down song may be heard at the wrong time compared to the original song. The error is generally only some milliseconds, but that's enough to make your timing inaccurate if you time a song at a slower speed; most commonly, this will mean that your offset is wrong. The exact error is also inconsistent between different sounds and impossible to predict, so osu! can't automatically correct it.
Therefore, disabling playback rate adjustment in the timing menu was the only way the devs could reasonably address this issue. This way, people won't get the wrong timing because they mistakenly trusted an inaccurate version of the song.
As a side note, the issue is also present in stable. However, as far as I know, it wasn't noticed/acknowledged until stable was already basically not getting any new features, so playback rate adjustment remains available despite the issue.