To be fair, unallocated or unassigned IP space isn't fair game to use for testing outside of an air gapped lab. I've never in my career thought it would be a good idea to "test" unallocated public unicast address space on my edge routers.

  On an airgapped lab it is bad practice. Same -though DNS related- with using *.local as a LAN TLD

We just should not.

There are instances where completely replicating an environment in an air gapped lab is acceptable. For instance, you could be encountering some edge case where a bit is being flipped in an IP address, or a bit is stuck on somewhere and replicating the environment down to the individual IP addresses plays a huge part in reproducing the issue.

An edge router is not one of these cases though.

.local is perfectly fine according to the IETF https://tools.ietf.org/html/rfc6762

For mDNS, yes. There are resolvers that don't even try DNS when looking up a name in .local.

Some of them were made more lenient because just as Microsoft finally stopped encouraging this nonsense, the Kubernetes people picked it up from ancient Windows Server folklore and apparently decided to make it web-scale.

As you linked, it is reserved for mDNS.

It was assigned to APNIC in 2010 http://seclists.org/nanog/2010/Jan/776

Allocated, not assigned.

APNIC used that terminology in their analysis paper of 1.0.0.0/8, "1.0.0.0/8 has been assigned by IANA to APNIC on the 19th January 2010" [1]. To do this analysis, wouldn't they have had to advertise a route back in 2010?

[1] http://www.potaroo.net/studies/1slash8/1slash8.html

What is the practical difference?

From RIPE, "An allocation is the block of IP addresses that is reserved by the RIPE NCC for your use now and in the future. An assignment is a block of IP addresses from your allocation that is used on an active network."

https://www.ripe.net/manage-ips-and-asns/resource-management...

I was curious as well and found this:

https://serverfault.com/questions/191497/ip-addresses-assign...

It didn’t do the bad thing until after it was assigned, so no one cared when it was allocated.