That is a sentence fragment. It should not have a period at the end. I also don't think the 'A' should be capitalized.
Respectfully, I think I disagree.
Lack of capitalization here looks wrong, as does the lack of a period.
This [issue] feels to me like one where a native speaker recognizes the capitalized blurb and period as correct, but a non-native speaker working off of perceived language rules rather than an ingrained experience of how language is used comes to a different conclusion.
Based on formal style guides, I believe their suggestion would be that the subtitle should also be in Title Case, giving
Rust: A Language Empowering Everyone to Build Reliable and Efficient Software
but using Title Case for subtitles has almost completely fallen out of favor (somewhat) recently. Instead, sentence case seems to be the common practice here.
If the subtitle were more visually offset, such as with, say, small caps, I might agree with you that the final period would not be necessary or desired.
However, provided that it's styled the same as any other text and not visually marked specially, the subtitle reads as a sentence with an implicit "[Rust is]" before it, and should thus be styled as a sentence.
More generally, web publications that aren't journalism tend to shy away from the formally correct This Reporter Found Multiple Interesting Factoids They Think You Will Tolerate Ads To Learn, and the more informal sentence structure is predominantly used instead.
I’m not sure the premise of this proposal is correct: That only full sentences get capitalization and a period. I really don’t think that’s true. Really. Any disagreement with that? After all this is English. A natural language without a former grammar whose “rules” at times are presented over-simplified to kids when they’re first learning it. They’d never be wrong with only writing full sentences. A good rule of thumb, sure. No reason for turning it around and disallowing everything else, though.
Whatever the choice, please limit the visual clutter when reading by keeping almost all letters lower-case. Some of us grew up in the era of upper-case-only line printers; we welcomed the change when output devices became capable of printing in other than SCREAMING CASE. As a professional editor for over 15k pages of IEC standards, I have concluded that Title Case Impedes Readability.
As a native English speaker, I subjectively think the capitalization and period look wrong, and I objectively know that they are grammatically wrong. I think removing the capitalization and period removes the pretense that it's grammatically correct.
Moderator note: This probably won’t be settled by debate or vote here. Perhaps this would be better as an issue or pull request on the repo suggesting a good style guide to follow, and then the maintainers can decide whether to adopt that guide (or possibly another one).
Let's look at what do other programming languages do. I'm too lazy for screenshots, feel free to suggest more/other languages if you miss any.
Proper full sentence(s): Go, D, Php, Cyclone, F#, CoffeeScript, TypeScript
Full sentence, but missing period: OCaml, Dart
No full sentences below here.
No period, but capitalized: Erlang, Haskell, Mathematica, Groovy
No period, but capitalized - in one line with the language's name (with a seperator in between) as a heading for a longer description: LaTeX, Idris
Period and capitalized: Rust, Kotlin, Swift, Elm, Toml
ALL CAPS: Ruby
Title Case: Matlab
(just a very minor editor nit: there is a difference between
font-variant:small-caps (small caps) and
text-transform:uppercase (all caps). Ruby uses all caps; my example above uses the (imho strictly superior (with font support) (but still probably ill advisable)) small caps.)