Stand-alones have grown in popularity especially with publishers becoming more and more risk adverse. The likes of Gaiman, Travis Baldree, TJ Klune all have great stand-alone vibe.
Having said that you are right that, especially for epic fantasy, multiple books are more common but i think that’s more trilogies etc than series - in fact I’m not sure fantasy has a big lean towards series (whereas perhaps sci-fi has a lot of series and comparatively less trilogies etc?)
Virtually every major genre defining piece of fantasy fiction/worldbuilding between LotR's massive surge in popularity and Broken Earth is, at this point, more than a trilogy. A lot started as trilogies, but got built on. There's more standalones than trilogies at this point in the 800lb gorilla section of the room. I think the best you can get for legit unlinked major trilogies in this time period is... The Fionavar Tapestry and Nevernight? I'm sure I'm missing some but probably not enough to change the point. Actually I guess Gentlemen Barstewards and Kingkiller are technically trilogies for now, but that's done neither man's reputation any good.
And given how series get built out, the likelihood of Broken Earth and Green Bones and The Poppy War and Tide Child staying trilogies seems small. Yeah, most fantasy authors seem to be given a trilogy to start with at the moment (at least the ones they have confidence in), but they get linked up.
Not that I see a point in dividing series and trilogies, particularly not when the point is the fantasy genre doesn't get on well with standalones. The exact type of series doesn't matter. Just that it's not a standalone.
And yes, standalones get published. Yes, many do well, in some cases very well, and their authors do very well with them too. I'm pretty sure all of them would have done better for focusing more on series.
Most of those doing greatly mainly on standalones either built their rep with a series (Gaiman) or are going to turn their breakthrough standalone into a series (Baldree). The only three authors I can think of right now who are genuinely big names and have done it all with standalones are Susannah Clarke, Madeleine Miller, and maybe Erin Morgernstern. Whether they stay like that, how their reputation survives - that remains to be seen. Right now they're unicorns. I suspect all of them would be a bigger deal with series.
And as such... well, I know it's the given advice. I know publishing has its reasons. But as fantasy stands, I don't see why authors shouldn't just plan for a series, even if it's a loose one. Lie to the agent if you have to, although I would ask agents why are we bothering with the facade. But, much as I don't want to see standalones die out, may the deity of their choice bless any author who's not planning on a series in this fantasy genre, because they'll need it.
