...to wonder where some topics disappear to?

[RuleBritannia]

I enjoyed reading Embarrassing Things That Children Said and Nice Things That Strangers Did but I can't find them now they've gone from the list on the right.

I haven't looked for it, but I wonder if it got moved to Mumsnet Classics?

www.mumsnet.com/Talk/_chat/1575113-The-embarassing-secrets-your-DC-tell-other-people-about-you

'Nice strangers' is probably a few pages back on Chat I would have thought.

They don't stay on the list on the right for long anyway - I assume you mean DOTD and Most Active? DOTD gets changed as and when MNHQ see fit and Most Active relies on the thread being active.