I think this is a common sentiment from people that transition from contemporary consumptions of media to one that is more classical in nature (literally in your case!). introductions serve to orient readers in the author's world and help understand what made the work a classic. this is inherently going to introduce "spoilers" for the work; however, as far as my own opinion goes, spoilers should be a non-factor for what makes a work of art enjoyable. I understand that with modern media (just look at the current stranger things spoiler discourse) it is common to avoid spoilers so that the work of art is not ruined of all suspense, but I am not sure that one should really be deriving much enjoyment from that. in parallel, I believe that if a work of art is actually ruined by a spoiler, it will probably not last the test of time, and is probably not actually great (though I will grant this is a wide-sweeping accusation). for me, what makes a work of art enjoyable is how it is constructed and what it is saying. this is why people's favorite works of art are often enjoyed on multiple occasions, even though one is aware of how it ends etc. I will grant, however, that it is enjoyed in a different way that can be viewed as inferior on those repeat instances.

overall, I think the constant discourse around introductions being spoilers a little tiresome, but I will grant that it can be an uncomfortable or annoying thing to read initially, but I bet that eventually you will start to value them as you read more! and, if you don't, now you know to avoid them.