I haven't found a clear and updated answer, even after googling for a few hours, so here it goes:
I am aware of the advantages and disadvantages of both Microdata and JSON-LD. I also know that Microdata was dropped from W3C (and consequently from the browsers' API). What I'm not sure about is that how it will affect any site where Microdata is used specifically for SEO purpose.
Does Google support JSON-LD for SERPs? What format does it recommend to use? I am looking for updated answers - not from 2011 or 2012 (if they are still applicable though, feel free to post it).
What is more appropriate for a dynamic site with lots of contents (think: 50000 videos, images etc): JSON-LD, Microdata or RDFa? Why?
Consumers that support Microdata support Microdata, no matter if or where Microdata is specified.
It’s conceivable that new consumers might decide not to support it, but the syntax is still very popular and still part of WHATWG’s HTML Living Standard, so it’s probably not going to vanish.
Some years ago, JSON-LD was not supported for many of their features, and they recommended that authors use Microdata (and they supported RDFa, too). Today it’s different.
See Google’s Markup formats and placement:
JSON-LD is the recommended format. Google is in the process of adding JSON-LD support for all markup-powered features. The table below lists the exceptions to this. We recommend using JSON-LD where possible.
According to the mentioned table, Microdata and RDFa support all of Google’s data types, while JSON-LD supports everything except their Breadcrumbs feature.
I wouldn’t give much weight to their recommendation. They say that "Structured data markup is most easily represented in JSON-LD format", but I think it’s safe to say that this only applies to authors that generate the structured data programmatically (especially from tools that support JSON).
For authors that manually add the structured data markup, it’s typically easier to use Microdata or RDFa, and using these syntaxes minimizes the risk that an author updates the content without updating the structured data, too (see DRY principle).
Unless you know (and care for) consumers that don’t support all three syntaxes, it doesn’t matter. Use what is easier for you and your tools.
If you have no preference, I would say JSON-LD or RDFa, because contrary to Microdata,
JSON-LD if you like your structured data not "intermingled" with your markup (= duplicating the content), RDFa if you like to use your existing markup (= not duplicating the content).