I had noticed the same on my reading list - lot of missing articles.

Perhaps coincidently, after 1.5 years of paid membership, medium gave me a notice last week that my account is in violation of their policies because I republish my articles on medium. One of the biggest value that I had found with medium was that I can track how different audience behave depending on when and where I share my stories - different social media, different publications, etc.. I use multiple copies for this purpose.

My audience engagement is now spread across many copies, which obviously I can't choose between. Medium hasn't deleted any of my copies, but if they do, then it's likely that is one explanation of missing articles.

Readers of my popular articles are mostly external non-paying users. So unsure if non-monetization would fend off any cleanup bots. Would appreciate any ideas.. I haven't got any feedback from medium admins.. I like the layout of medium, so wasn't keen on using wordpress or alternatives, but I may be forced to if medium starts deleting..