When you say tabular data, do you actually mean serialized data?

See https://www.mediawiki.org/wiki/Extension:JsonConfig/Tabular

I'm not sure if T28059: Add support for KML/KMZ filetype is closely enough related to this, that it ought to be mentioned and considered as another specific example in T120452?
I do see that there are some complicating factors regarding security with KML/KMZ (see comments from Chris and bawolff), so possibly that excludes it. Or possibly (likely!) I don't know enough about these file types at all. :) Thought I'd mention it though, just in case.

@Yurik, could you provide an update on this?

I have spoken with @daniel to sort out any concerns from Wikidata. Overall, there were no objections, and I will implement string size constraint of 400 chars (not bytes). nor match /^\s|[\r\n\t]|\s$/ regex (or stricter). My next step is to revisit the various proposed tabular json schemas before enabling it.

Update: spoke several times with @brion about this, everything seems to be a go. The current implementation has been live on beta-cluster for over a month, e.g. here. All documentation is now on mediawiki.