I often read older science fiction and fantasy, for several reasons: because I'm interested in the development of the genre, I liked an author's current books and wanted to seek out their backlist, or there's something particularly appealing about a particular author's older work

The conventions of the genre have changed a lot over time, from how race and gender and sexuality are ignored or foregrounded, how romance and consensual sex are depicted or not, how rape and violence and slavery are depicted or not, how exposition and pacing and stakes are used

You are more likely to find rape, attempted rape, and slavery used as plot devices without examination and without the blurb indicating these things will be part of the story in older SFF, more likely to find vaguely or even overtly racist and Orientalist worldbuilding

Not all older SFF had all of this and it gives cover to the worst of it to say it was all like this or a product of its time, but much of it has at least some of these problems up to a decade or so ago, and having to steel yourself for it is rightly a dealbreaker for many people

Older SFF, especially older fantasy, is more likely to be set in worlds that are all white and never queer, in a few stock settings like high fantasy's vaguely medieval Europe or "exotic" amalgamations of different Asian cultures, and this is rightly a dealbreaker for many people

More neutrally, the style of much older SFF is quite different than much current SFF- older books may be less streamlined, more prone to digressions of worldbuilding or description, with offscreen or no sex and airbrushed or glorified violence and those are valid dealbreakers too

When I recommend older SFF to someone who hasn't read it, I first consider whether these things are likely to be hard dealbreakers for them before I recommend it at all, and if I decide they're not I try to qualify for them as best I can at the time I recommend it

(I may not be able to do this well, if it's been a long time since I've read it or if I don't have the spare executive function to work through what I read past that might be a dealbreaker for someone else or if I know about a book but haven't read it myself, and I try to say so)

No one is obligated to read older science fiction and fantasy, though, and there are lot of reasons it might be a dealbreaker for some people- the good news is that people have pushed the field to do better about a lot of the non-style-related dealbreakers, and much of it has!

I read a lot of new science fiction and fantasy and I find it's a worthy continuation of the older science fiction and fantasy I like, I feel like the future of the genre is bright- if you only want to read newer work, that's a valid choice and there are great books awaiting you!

If you'd like some recommendations with qualifications for older SFF, especially the older work from the erased period from the 80s to the present that is a more direct influence on new SFF than Campbell, Heinlein, Asimov, etc, I have some for you here https://twitter.com/quartzen/status/1212146642239000581?s=21

Addendum: When I talk about dealbreakers of content or style being present to a degree in much older SFF, that absolutely applies, as Jeannette Ng astutely points out in this thread you should also read, to authors who were progressive for their time too https://twitter.com/jeannette_ng/status/1289890572296007680?s=21