I dont have any research to cite in this reply, but I’ll say this.  what are the goals of some of these spinal flexion exercises? if it is “strong core” ie. situps, crunches, i would argue that it can create a lot of hypertrophy for looking good in a bodybuilding competition but if it’s strength then there are better ways of building bracing that do not flex the spine in ways that may result in injury.