I dislike the modern left as much as anyone in Hanania's comment section and so I'd like to believe that the core of something like Marxism is a baseless conspiracy theory, but honestly, from what I know of it, that doesn't seem to be the case. I'm not saying it's correct necessarily but it is not a conspiracy theory as I would understand the term.

It does assert that a class of capitalist elites collectively oppress an ordinary, working-class proletariat, yes, but not via a conspiracy. The charge is not that they actively conspire with one another to plot against the working class, but that they are all subject to similar material incentives that push them towards independently taking the same exploitative actions in order to compete with one another. If one capitalist pays his workers $2.50/hr and the other $1.50/hr, all else equal the latter will outcompete the former, which forces the former to also lower his workers' wages to the same level to stay in business. That's a simplification obviously but those kinds of ideas are what it's about. It's actually not even a choice capitalists are making to exploit workers, necessarily, although they do benefit from it of course; they couldn't choose not to exploit workers even if they wanted to.

Other leftist ideas like structural racism similarly do not necessarily assume an intentional, coordinated effort to oppress this or that group is required for people to do so.

Contrast this with a more classic conspiracy theory like that detailed in the Protocols of the Elders of Zion, which purports to be a record of a meeting of Jewish elites in which they discuss an intentional plan to subvert and oppress gentiles. Even if you would consider both Marxist ideas and the Protocols to be examples of conspiracy theories I think you can see at least that they are not of the same kind.

That said, there are actual conspiracy theories that, true or false, leftists have endorsed in the past, such as the Business Plot, in which it was alleged a group of wealthy businessmen were actively conspiring to conduct a coup and install a pro-business Fascist regime in the US, or the Doctors' Plot in the USSR. I would consider those to be examples of actual leftist conspiracy theories, but concepts like Marxism, Patriarchy and what have you, true or false, do not meet my definition of conspiracy theory.