My suspicion has been that the well-publicized removal of many servers that used to be part of Twitter's infra had a negative performance impact, and the series

My suspicion has been that the well-publicized removal of many servers that used to be part of Twitter's infra had a negative performance impact, and the series of changes to how anonymous users and/or tweet embeds worked are a band-aid on that. For an anonymous viewer, they still don't show you the rest of a thread a tweet is in, or even any indication of the fact that there is a thread that you're not seeing. This doesn't even make sense as a nudge to join Twitter, so load reduction feels like the most likely reason to me.