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?
These are surprising facts to know. How come?
Esperanto came out in 1887 and there wasn't many good conlangs that time. A few decades later communism became the greatest hit in Europe so it got overhyped because of the idea of one universal language. When the hype went down, people realized that it's not a really good conlang to be used as an IAL because it has many unnecessary features (like gender specific pronouns) and that it's very eurocentric. The vocabulary is mostly just English and French words with a slightly changed pronunciation and the grammar really resembles indo-european languages as well.