That's a weird statement to make though. I would say that it was Gentoo's decision to test for all functionality.
But, even ignoring this for the moment: how does this apply to the linux kernel? Because the config file has TONS of options. Does Gentoo support all of these via use flags? If not, why not? Because that distinction to me would be totally arbitrary.
Because Gentoo has basically three options; let me know if you can think of a fourth option I'm missing.
1. Turn every optional feature from upstream into a USE flag, and test them. Which means testing all the 2^N combinations.
2. Turn every optional feature from upstream into a USE flag, but do not test them. Which means users can end up with untested combinations, some of which have proven in the past to be buggy.
3. Stop turning every optional feature from upstream into a USE flag. Which means giving up on Gentoo's distinguishing characteristics for this package.
Okay, I guess there's a fourth option:
4. Stop packaging the Chromium package at all. Which is what they've ended up doing.
These days there is a package that builds the kernel for you, but it relies on the "savedconfig" system to just save/load the entire kernel config file instead of using USE flags. So you still have to root around in the config menu yourself to change things.
In my experience Linux is very good at correctly declaring dependencies between options, and still working with options disabled, so this isn't as much of a problem as it seems to be for Chromium.
They're free to limit which options they offer, they're free to limit which combinations they support, and they're free to fork or drop the software they offer if they insist on keeping the 2^N purity (which they eventually did). But that's Gentoo's mission, not upstream's. Upstream is just giving them software for free on an as-is basis.
While the "we're entitled to bugfixes" attitude is definitely annoying for software maintainers to deal with, and you (as the end user) aren't entitled to anything from 99% of open-source projects... this is Chromium we're talking about. The browser engine that powers 75-95% of all Web browsing (it's very hard to pin down actual numbers, but it's huge). It is not too much to ask that the Chromium maintainers either 1) test all the combinations of optional flags they have, or 2) declare that some of those optional flags are no longer optional. In other words, to act like professionals.
(I don't use Chromium myself — switched to Firefox years ago because I tend to open a lot of tabs, which make Chromium seem to just gobble up RAM, and Firefox appears a little better at RAM management, or at least it did at the time I switched (haven't checked recently). So this isn't "the Chromium team owes me something", this is shaking my head in disappointment at what the Gentoo maintainers report that Chromium team is doing, or rather not doing. As I said, there are some projects — the Linux kernel is one, and Chromium is another — that are so widely used that a certain level of professionalism should be expected of their maintainers. I agree that that attitude of entitlement is not deserved in about 99% of open-source projects. But Chromium is part of the 1% exception, IMHO).
I just don't like this idea that downstream maintainers need to be saved from themselves by limiting the options upstream provides them.
Downstream are not end users, which I agree should only get tested builds. They're professionals too, and they can be trusted to pick the configurations they want to build, evaluate how much more testing they need, and cut out configurations from their support matrix if they don't have the resources to support them.
Also, this idea that a professional project wouldn't offer untested combinations of settings to other professionals is why people follow up asking about Linux. A quick search tells me it's got ~19,000 Kconfig options[1].
[1]: https://dl.acm.org/doi/fullHtml/10.1145/3646548.3676548