Technically twitch already has this feature, but it's not on tags, but on language settings. When I change the twitch language, the streams that has the language tag that matches the viewer language setting are promoted to the first row.
I wonder if it can be generalized to other tags - e.g., have a list of highlighted tags in setting so that the streams that contains any of these tags are promoted to the first row. The language tag could be the default one, so it doesn't change anything if you don't modify it.
This is different from filtering by tag, because filtering by tag will hide all other streams. And filtering by tag doesn't stick, it doesn't show up when you switch to a different category, unless it's a language tag which remains permanently and shows up on all categories (which is quite unnecessary because there's already language tag promotion).
So, I think having a list of highlighted tags would be great for viewers that have a preference for certain tags.
And the tags that match the highlighted ones under the promoted channels can have a different background color to show what highlighted tags they have, which makes it more clear.