Just wondering because just about everything I have bought so far has the special features on blu ray discs?

Very few, like a handful if that. Most just add the special features disc from a previous release into the UHD set. They don't remaster them for 4k or anything like that.

There just aren't many special features even filmed in 4K. Budgets on special features have been drastically scaled back with physical media shrinking.

I still can't believe there is virtually nothing concerning music or music concerts on UHD and special features also in UHD.

Movies are somewhat limiting the field as far as genre's are concerned and if they think UHD will ever become popular then they (the movie industry) are in for a long wait.

I know I've got at least one with 4K features, and several where the 4K disc at least still has the extras on it in 2k (or whatever they call it now.)

There are more than you'd think. The list on blu-ray.com's forums includes 42 titles (!).

As the first few posts in that thread confirm, they just ended up listing 4K discs that have any special features included, not 4K discs with special features in actual 4K resolution. I'm not really aware of any discs that have actual 4K special features (there may be one or two). Pretty much all of those special features are in SD or HD.

Including The Wizard of Oz, which has the standard-def TV special with Angela Lansbury (included on several previous releases) upscaled to 1080 but then incorrectly encoded at 24fps,
making it look worse than it does on the VHS.

Just picked up 1917 on disc and all the extras are in 4k. Very nice.

The OP definitely got a lot wrong when it first went up, but it’s continually updated, whereas the posts you're referencing are a year and a half old. Many/most of the incorrectly listed titles were removed before I posted that link. It also doesn't mean all/most extras are in 4K...just something, and usually more than a teaser or trailer.

You can click into the reviews and see which titles have extras in 4K. 1917 has 5, for instance. BlacKkKlansman has 2. The Grinch has 16 (!). The Fifth Element has "The Director's Notes: Luc Besson Looks Back", which is in 4K. The Karate Kid has "Remembering The Karate Kid with Ralph Macchio, Billy Zabka & Martin Kove". Apocalypse Now is a little messier since they're counting alternate cuts as extras, but nothing else is in UHD.

It's also not a complete list. For instance, It's a Wonderful Life has a restoration featurette in 4K (and Dolby Vision!).