The original spirit behind the requirement to register meta[name] extensions was
good, but in practice it has ended up costing a large of maintenance time
without it being clear how much real benefit to end users it has provided.

Many values listed at https://wiki.whatwg.org/wiki/MetaExtensions appear to be
essentially for private use for only the people/organizations who minted them,
without it being clear how much general utility they might have, and without any
evidence of public applications that consume them.

So this change drops the requirement to formally register new meta[name] names,
while still retaining a reference to https://wiki.whatwg.org/wiki/MetaExtensions
informatively—in order to encourage authors to consult that before creating and
using any new meta[name] names.