- Neither Linux nor Windows provide one, due to licensing costs
- Most browsers do not deem it worth the risk of relying on host software decoders, as the plug&play infrastructure behind them usually translates into "nobody feels responsible for patching anything" which translates into incalculable vulnerabilities
That leaves MacOS (why bother when Safari exists), iOS (dito), and Android (why bother when Android users don't spend money), and trying to use hardware codecs without stepping on patents enough to make lawyers smell blood (why bother).
Some hardware comes with the codec pre-purchased (my laptop does). I don't think I've ever heard of anyone buying a codec in the MS Store, though.
I think most Android phones come with HEVC support. Mine did, anyway; I suppose it's up to the vendor to choose if they want to support it. Apps like VLC will also play the codec just fine. My camera app even records in h.265. This is a phone from Xiaomi, which not exactly known for their great software packages and compatibility.
I'm pretty sure Safari already supports h.265 because Apple switched to HEIF pictures while the rest of the world still just uses JPEGs for everything, and HEIF is pretty close to a single h.265 frame packaged as a picture. Not even Apple would be so foolish to switch default formats on their mobile devices and not support it across their software products.
Browsers don't feel like paying license fees over downloads of their free products and I can't blame them. Mozilla's h.264 decoder is only published along with it because Cisco had reached the license fee cap (which doesn't exist for h.265) and they decided to use their license so that Firefox can play videos freely.
I think Apple and physical disks formats are the only players heavily invested in h.265 right now. AV1 hardware decoding support is slowly coming along, so soon enough everyone can just use AV1 and be free of the proprietary patent bullcrap.
Mobile support comes from the chips (SOC) that the phones are using. Hardware codec support is a major differentiating factor for mobile chips. It has a huge impact on power usage, e.g. decoding video or audio in hardware could be 10x more efficient.
It's common that the manufacturer doesn't actually get a patent license, they get "indemnity". If someone were to sue you, the chip manufacturer would handle it, because they have their own patents and have cross-licensed with the others in the pool. You may have to pay the chip manufacturer for the indemnity in addition to the chip costs, i.e. "paying protection money". "That's a nice restaurant you have there, it would be a shame if someone set it on fire. We can protect you from bad people like that."
> I think most Android phones come with HEVC support. Mine did, anyway; I suppose it's up to the vendor to choose if they want to support it.
The official requirements for Android phones can be found on the Android CDD (https://source.android.com/compatibility/cdd). From a quick look at the CDD for Android 12, section 2.2.2 says that the required codecs for encoding are "H.264 AVC" and "VP8", and for decoding are "H.264 AVC", "H.265 HEVC, "MPEG-4 SP", "VP8", and "VP9". So it seems that all Android 12 phones will come with HEVC decoding support, but not necessarily encoding support. Looking at past CDDs, it seems that "H.263" was required for encoding and decoding before Android 7.0, and "H.265 HEVC" was required for decoding starting with Android 5.0.
- Most browsers do not deem it worth the risk of relying on host software decoders, as the plug&play infrastructure behind them usually translates into "nobody feels responsible for patching anything" which translates into incalculable vulnerabilities
That leaves MacOS (why bother when Safari exists), iOS (dito), and Android (why bother when Android users don't spend money), and trying to use hardware codecs without stepping on patents enough to make lawyers smell blood (why bother).