PSA: You do NOT need to download the fresh Not Fucking Dead With MAP Developer Properties Fix upload if you're not a developer actively modifying the HammUEr source to integrate it with whatever flow you're trying to build for your project and you're importing some kind of customized MAP format... descendant.
You don't. There's absolutely no changes that are useful to you if you're just importing your levels. You're fine. Go about your business and just ignore that upload.
The only change is that I fixed an oversight where VMF entity properties were queryable during the Load-And-Spawn portion of the HammUEr import so devs could do whatever for their custom extra properties... but I never hooked up the same code for the MAP loader. Which I discovered when someone asked me about accessing properties so they could do something custom with entities, and I went "I vaguely remember doing something for this, can't you just call this functio... oh. Huh. I never hooked that up for MAP files, did I?"
That's all.
(Note that if you're a dev that does want that but isn't on 5.5, you should be able to just take the HammUEr lib and paste that over your existing NotFuckingDead install, rebuild and get this oversight fixed as well)