If a tag is synonymised to a master tag:
Example> The master tag is music, a synonymised tag is jazz.
If people are searching jazz on the internet, does the jazz tag still catch search engines?
I was able to prove as such with a basic proof of concept; by searching Google, but limiting my searches to hits specific to the exchange site I knew used the phrase as a synonym; the collision-testing synyonm, at the Game Development Stack Exchange, which links to the master tag of collision-detection.
More specifically, I searched for "collision-testing", while omitting "collision testing"; that is, I removed hits that were returning false positives because the question, itself, contained the phrase "collision testing".
The TL;DR answer, with regards to tags, is generally NO
The actual answer is a bit more complex...
302 redirect (not a 301!) to the JS tag. Both have the same net effect: search engines will de-index the page because it's returning a 301/302 redirect, and not a 200 status indicating content. This is actually desired behavior because it would be really annoying to have search results link to a redirect page instead of the end page.
What's odd is that SO/SE isn't using a 301, which is a permanent redirect. Maybe that's for the off-chance that someone will remove the synonym, but in practice it makes for poor indexing. I mean, accepted SEO practice is to use a 301 redirect
Some of Google's employees have indicated that there are cases where 301s and 302s may be treated similarly, but our evidence suggests that the safest way to ensure search engines and browsers of all kinds give full credit is to use a 301 when permanently redirecting URLs. The Internet runs on a protocol called HyperText Transfer Protocol (HTTP) which dictates how URLs work. It has two major versions, 1.0 and 1.1. In the first version, 302 referred to the status code "Moved Temporarily." This was changed in version 1.1 to mean "Found."
A 302 means that the page might come back so keep coming here. Unless there's some internal metric showing a lot of synonym removals, I would suggest these be 301s, especially since there's no indexing of 302s anyways. Would help some with SEO (the concept is called crawl budget)
Why bother, you ask? And what does this have to do with the question? The reason is this:
Going back to my ECMA example, the JS tag is #10 on Google for JS. But this question is #9 for ecmascript. The tag never shows, but SO still shows up as relevant because it has a good page on the subject. In other words, the synonym tag is nowhere to be found, but relevant questions are still indexed and ranked. I'd rather see someone find an SO/SE page that answers a question, than to show them a potentially confusing tag page that just lists questions. I'm willing to bet most people come directly to a question anyways, as opposed to a tag.