I don't like the term legacy, I prefer "Classic". I like classic because that's what it is. Wing Commander was a "Classic" game. It still works, but it hasn't aged well compared to some new games. If you're working with the Notes Client the LotusScript is still very relevant. It's your bread and butter and always will be for the Notes Client.
If you're working in XPages then LotusScript is a lot less relevant, but still has uses. Like an aging sports hero it's not the key player that it once was.
But here's the thing, do NOT stop learning with SSJS. You want your final destination to be Java. Java sucked in Notes Client. You could only use it in Agents. So few people learned to use that. There was no incentive to use Java in Client apps. XPages is different. You can use Java everywhere! This is not to say that you need to use Java to build an XPages application. You don't. But Java and these Managed Beans is a really really good place to put your logic. Wouldn't it be nice to have a LotusScript custom Object that could live between changing forms? Just be Global? Well that's what Java Beans are in XPages. It's a really good and powerful way to work.
Even in the XPages world LotusScript is still used for scheduled agents. But that might actually not be needed at some point now that we have DOTS - which appear to be much faster and more powerful then agents.