Redirecting one or more domains to your main domain for any reason other than 1. an effort to consolidate multiple, similar websites or 2. changing the primary domain name of your website is a bad practice, and could lead to being penalized by Google. This has been known for the better half of a decade. For example, back in 2011, JCPenny learned this the hard way, which was widely publicized.
As far as Google understanding the use of numbers in a domain name, such as "4 = for" "2 = to" etc; I certainly would not underestimate Google's ability to understand the contextual meaning.
Additionally, just because a domain isn't indexed doesn't mean that it is dead weight, or that they don't have some intrinsic value, but for the purposes that you outlined, the most prudent advice that I can provide is that they have no use. To further illustrate this point, check out this post in 2015, which specifically states that Keywords in a TLD do not give any advantage or disadvantage in search results.
Hopefully, this helps. Let me know if you would like any clarity.