You can also have an input with a defined delimiter (such as comma, semicolon, etc) to separate entries for exploding on the backend.
HTML doesn’t support dynamic ones natively because it is built for generating mostly static controls. Do to an array input “properly” you’d need to have to dynamically generate new input nodes in a consistent way and name pattern. It’s not impossible, but it is low-hanging fruit on the grander scheme of the HTML spec currently.
HTML does support a static length array input already. You simply provide the input options and name them with
name="arrayname" and at least in the case of PHP (and most of web frameworks I see) they will instantly combine those entries into an array. So if you don’t need a dynamic number of entries, it’s already available.
Since the web platform already allows for both static and dynamic (with JS) array input options, it isn’t something the standards people are too interested in doing. As per the Extensible Web Manifesto they are looking more into new features that add capabilities that can’t be achieved or are extremely difficult and/or costly to do currently.
Your best path forward to get a new element for this made is to build your own. Make a web component that encapsulates your idea and can be used in production apps for experimentation. Once it has gained traction and you’ve worked some of the integration issues out that will exist, you then have better grounding to push to have it standardized and provided in user agents.