There is undoubtedly a ton of socialist history regarding the promulgation and study of Esperanto.
https://en.prolewiki.org/wiki/Esperanto
It does not seem like it is nearly as popular as it once was, but there are examples of it being used and even celebrated in Cuba, China, and the DPRK.
I adore the idea of a lingua universalis. I am also aware of many of the criticisms of Esperanto, from its ostensible Eurocentricity to its difficulty with escaping unnecessarily gendered language.
Is there much use in learning it, outside of personal interest or as a hobby? Do you think that there are Esperantist movements large enough to justify learning it? Enough speakers?
If you want a serious answer, no, I don’t think it’s worth it.
Learning Esperanto means you learn one of the very first attempts to make a universal language from scratch (first doesn’t mean good), a very eurocentric language as its critics have put it, and you basically sell your soul to the Akademio, the entity regulating Esperanto. I don’t see anything to gain from that.
The fact that communist countries have a history with it and celebrate it seems to be for different reasons. For example, the USSR once tried to use Esperanto because for them it was better than using English, which was the language the USA were pushing forward, a country that used capitalism as its model. Not to mention that at the time, there wasn’t wider knowledge of which languages were better than Esperanto, the popular “universal language”; it’s no wonder things happened this way.