LineageOS's supported devices are typically a bit older (e.g. Galaxy S6 but not, say, the S10). But support will likely come after some time.
Meanwhile, there's this new thing called Treble, where you can flash a generic ROM on any new Android phone. There isn't an official LOS Treble build (for various reasons), but it's a solution if you must have a 3rd party OS build.
Well, my Pixel XL is a bit older, and I happily run 9/16.0. Just can't find if it's planned to be lifted to 10/17.1.
I never buy one that can't run (any version) of LineageOS right now though. That's why I got this older Pixel in the first place. Its' the only way to get some sort of longterm updates and a degooglified Android.
OnePlus makes really good phones for good prices, and tends to have great support from Lineage. Only downside is that it's a Chinese brand. Of course you're replacing the OS wholesale, which would get rid of any traditional telemetry, but technically it's possible there's something nefarious in the firmware itself, depending on how paranoid you are.
Looks like the gsi (general system image) that vendors need to boot to demonstrate treble compatability - is mostly tested for exactly just booting/starting. So any phone that supports treble will boot gsi/boot an image derived from gsi - but not all phones will work with gsi - for a meaningful definition of "work as a full android phone".
In other words treble/gsi seems to be a bit of a half-measure - and if you "port" à given rom to treble/gsi - it still will only work on some subset of treble phones :'(
This is what I gleaned from some searching, anyway:
There's also annoying variations in GSI's - you need separate images not just for 32-bit vs. 64-bit but for A-only vs. A+B boot support, and sometimes for different "base AOSP" releases - GSI's for devices that shipped with Aosp versions 8.x, 9.x, 10.x are not always guaranteed to work consistently.
The Pocofone F1 is really cheap and really good. The only downside is the lack of NFC. I got my dad one last year with LOS as replacement for his Windows Phone which is no longer supported by WhatsApp.
Meanwhile, there's this new thing called Treble, where you can flash a generic ROM on any new Android phone. There isn't an official LOS Treble build (for various reasons), but it's a solution if you must have a 3rd party OS build.