I actually hit a bit of a wall trying to find new stuff in this niche recently. It seems like the real peak was a few years back with certain authors on platforms like Wattpad or specific adult story sites. A lot of the classics seem to revolve around age-gap dynamics or power-exchange relationships where the discipline is a core part of the contract, not just a random kink scene. The 'Forced Regression' trope is huge, obviously.
What makes a story stick for me, though, is when the humiliation or the caregiving feels psychologically real. If it's just a checklist of diaper changes and spankings, I lose interest fast. The tension has to come from the character's internal conflict—the shame battling with a weird sense of security. 'His Little Secret' by an author I can't recall nailed that push-pull for me, where the professional submissive character fights the regression every step but slowly breaks down. The popular ones often get the physical details right but miss the emotional weight, which is why I think they fade from memory.
I'd say the most popular ones are probably those that balance the taboo with a strangely domestic, almost slice-of-life feel in the middle chapters, before ramping the discipline back up.