11ty filter for returning a sorted list of tags from collections. Use the it in a template like {{ collections.foo | taglist }} to get the sorted tag list.

sort-tags.js

eleventyConfig.addFilter("taglist", function(collection) {
    const tags = [];
    collection.forEach(post => {
        tags.push(...post.data.tags);
    });
    const sorted = [...new Set(tags)].sort((a, b) => a.localeCompare(b));
    return sorted;
});