Optimally, we should have a flag for every legacy playback detail, rather than one global "More IT/FT2-compatible playback" switch. That way, some quirks could be re-used for other formats, and more importantly we'd have bigger control over achieving the same playback quality of a given module as in older OpenMPT versions.
For example, in general the new period-less linear slides mode for IT modules is of course great to have, but at high frequencies the differences between the old and new behaviour are so apparent that e.g. the drum line in znm-latenight.it at around 4:00 is off-beat. However, disabling the new behaviour for non-compatible mode would mean that we'd have inferior frequency control in modern MPTM files with disabled compatibility mode, which is not acceptable either.