ALSA has had support for software mixing for more than a decade now. Similarly, PulseAudio can work with ALSA to route any application that uses the ALSA API through PulseAudio.
AFAIK ALSA-only distros tend to have SW mixing configured by default and PulseAudio-based distros have ALSA intercepting enabled by default. So in practice an application that needs some basic audio playback can just use ALSA and it'll work everywhere.
> So in practice an application that needs some basic audio playback can just use ALSA and it'll work everywhere.
"In practice" doesn't mean what you think it means.
The behavior I described was with a Pinebook Pro which only came out about a year ago. That was running whatever version of Debian they shipped-- either stretch or buster, can't remember now. And there was no fancy or custom audio setup-- it was just an XFCE desktop environment with Pulse running however it's configured to run by default.
Also, as a maintainer of realtime audio FOSS software I can tell you users have reported these exact problems on other Linux distros. Over at least the last year and a half.
It appears that in Debian testing the configuration of Pulse "just works" as you describe. I would rankly speculate that Ubuntu 20.04 probably has a similar default setup. But to turn that rank speculation into "in practice" we'd need to actually test and see, and it sounds like neither of us have done that yet.
You wont find a sound setup that will work in all Linux distros, it is how the sound landscape in Linux is. However in terms of API availability, ALSA is available everywhere, is the "native" sound API and people can use it without installing anything extra. This is why i say 'in practice' - with anything else you'd need to rely on dependencies that may or may not exist. ALSA is the Linux sound system.
BTW what i describe is how Debian was for years when i used it by default, not just testing. It is possible that the version of Debian Pinebook Pro shipped was altered. Or something else wasn't configured as i had it, who knows.
I don't think that's important. In practice you can also use the pulseaudio API and that will work everywhere because of this: https://github.com/i-rinat/apulse
Apulse is much more cumbersome to use and not something you can really rely on - AFAIK it was originally written as a workaround for Firefox dropping ALSA support so you can still watch videos on YouTube, etc. Also isn't something that is installed system-wide, you need to explicitly call something through it.
AFAIK ALSA-only distros tend to have SW mixing configured by default and PulseAudio-based distros have ALSA intercepting enabled by default. So in practice an application that needs some basic audio playback can just use ALSA and it'll work everywhere.