.eleventy.js

module.exports = function(eleventyConfig) {
  // you’ll have to get a list of all the tags in your talks.json here which is not great but pretty easy
  ["Web Assembly"].forEach(function(tag) {
    eleventyConfig.addCollection(tag, function(collection) {
      return collection.getFilteredByTag("talks").filter(item => {
        // note here the use of the pagination alias `talk`
      	return (item.data.talk.tags || []).indexOf(tag) > -1;
      });
    });
  });
};

talk.njk

---
pagination:
    data: talks
    size: 1
    alias: talk
    addAllPagesToCollections: true
permalink: "talks/{{ talk.title | slug }}/"
tags: talks
---
{{ talk.title }}

talks.json

[
    {
        "event": "cascadiajs-2019",
        "speaker": "aaron-turner",
        "title": "Building for Budgets",
        "tags": [
            "Web Assembly"
        ],
        "slides": "https://docs.google.com/presentation/d/1S4vzn3FUMF2kT6GLaipjsb4pxmdPP14P3MVtFDEpXo4/edit?usp=sharing",
        "abstract": "A personal story of living in a low income family. With tips on how modern web technologies, like WebAssembly and Web Workers, can bring new experiences to the web for budget devices that struggled to run these experiences before.",
        "ytID": "CYVs9hllqZY"
    }
]