I have the 2020 Roku Ultra (model 4800X) and just updated it to OS 10.0.0 build 4177-C2. My TV, a 2020 model Vizio M55Q7-H1, supports both Dolby Vision & HDR10+ in addition to regular HDR10 & HLG. I installed the update right after learning OS 10 added HDR10+ support.
I then checked "Display type" under Settings, but "Auto detect" still shows only DV on my TV; the only mention of HDR10+ support there is under "4K HDR TV". Luckily, right after that I finally found the setting to disable always-on DV (the bane of Sony's 4K Blu-ray players where they seem to think it's a feature even though nearly everyone turns DV on & off as needed, one of the reasons I replaced my original UBP-X700 with a Panasonic UB820); IMO that should *NOT* default to on.
Then I tested certain movies on various streaming services to see if I could get HDR10+ streaming. So far, the only HDR10+ stream I've found is Alita: Battle Angel (a Movies Anywhere title so I have it several places) on Prime Video, a known HDR10+ supporter -- though I lucked out as Prime Video only flags HDR in general. The FandangoNow-powered Movie Store only played it in regular HDR10, though I understood FandangoNow itself supported HDR10+ (that could change now that Fandango also owns Vudu & many of us believe, even hope, that FandangoNow will eventually be merged into Vudu). Vudu & Apple TV, both known DV supporters, both play it in DV. The Movies Anywhere app, which also favors DV, doesn't give Alita any HDR at all; for some reason MA supports DV on all its 4K platforms *except* Roku.
Is there any way short of actual playback to confirm HDR10+ support in general on the 2020 Ultra when connected to an HDR10+-capable TV, especially if it also supports DV? (It may be easier on TVs that support HDR10+ but not DV, like most Samsung 4K TVs.)
Also, if a service supports both HDR10+ & DV on a title, which one will the 2020 Ultra choose? The Panasonic chooses HDR10+ which is closer to the capabilities of today's 4K TVs, though it's easy to disable HDR10+ in settings & force DV on dual-format titles (like the 4K Blu-ray version of Alita). The way the 2020 Ultra's settings are configured, it would seem the only way to force HDR10+ over DV is to dial "Display type" back to "4K HDR TV"; but if HDR10+ is the default on Roku (seemingly unlikely) there's apparently no way to force DV over HDR10+, except perhaps always-on DV.
Movies Anywhere supports DV on Roku (4800) since app update version 1.21.121 in early December.
Since HDR10+ isnt a selectable Display mode (neither is HLG for that matter), your only recourse is to play actual content (perhaps the only real utility value of the new "HDR Always on" setting is to verify its HDR output at boot).
As far as whether DV or HDR10+ is preferred - my guess is most app/service providers are going to prefer DV over HDR10+ (not that there are many offering HDR10+), but thats going to depend on the app provider and how the Roku is configured etc. Again, you'll just have to test to see how each app handles different configurations.
I tested playing the opening (generally just the studio logo) of numerous movies -- mostly with DV, but some with plain HDR10 -- in my MA app v. 1.22. In addition to Alita, the movies I tested this time included the entire Back to the Future, Christopher Nolan Batman (plus his Dunkirk & Tenet), Matrix *and* Lord of the Rings (original AND extended) trilogies, as well as *both* Mulans and three Pixar films, *plus* The Fifth Element & Crouching Tiger, Hidden Dragon (both plain HDR10) -- just to name a few. NONE of them played in *ANY* flavor of HDR; my Vizio TV kept showing "Normal" (SDR) even though it supports regular HDR10, HDR10+, DV & HLG.
If you're seeing DV in the MA app, it's almost certainly because you have the "HDR always on" setting at its default "On" -- which is *fake* DV on non-DV content (like my old Sony UBP-X700), *not* the real thing unless the underlying content is DV. When properly turned "Off", the Ultra only passes thru whatever HDR it gets from the streaming provider (much like non-Sony 4K Blu-ray players with the HDR formats they support on each disc, with higher-end Pannys like mine being the only ones that support all of them though I'm pretty sure HLG isn't found on *any* disc); that's how I got Alita in HDR10+ on Prime Video, DV on Vudu & Apple TV, plain HDR10 on FandangoNow / Movie Store, and apparently SDR on MA. (Basic HDR10 is mandatory on 4K Blu-ray; the only way you can get SDR is downconvert.) Apparently MA's Roku app doesn't support *ANY* flavor of HDR, much less DV. (OTOH, the MA app on my Vizio TV's SmartCast platform *DOES* support DV.)
I do agree with you that (a) it seems the only way to be sure which HDR is supported where is to play actual content in the various apps, and (b) streaming providers generally prefer DV over HDR10+.