I am assuming Neolemmix is the new standard that I should be creating levels for in this day and age?
NeoLemmix is by far the most popular engine for creating / playing content these days. Lix is also quite viable.
If you are going to use NeoLemmix, I'd highly recommend playing the NeoLemmix Introduction Pack before you start designing levels. You might decide you don't like the new features, and would prefer to just design levels for NeoLemmix using the classic features (or perhaps them plus a subset of the new features); this is perfectly fine, and I would say close to half of the available NeoLemmix content does something along these lines. However, it's good to at least know what they are and have at least some basic experience with how they work in practice - as well as some of the slight changes to how NeoLemmix works compared to other engines (in particular - bombers no longer have a countdown before exploding).
http://www.neolemmix.com/old/neolemmix/NeoLemmixIntroduction.nxp(NXP files are NeoLemmix packs; which can range from a few levels to packs the size of (or even larger than) the official games. You'll need
a copy of NeoLemmix to play them.)
Note that secret levels and gimmicks (except zombies) are going to be removed in the near future, so don't worry too much about those parts; and even more so, I'd advise not using them in your own content.