Strictly speaking, glue is only necessary in one scenario: nameservers falling within a zone cut defined by the same zone. RFC 1034 §4.2.1 makes this explicit, emphasis mine:
One of the goals of the zone structure is that any zone have all the
data required to set up communications with the name servers for any
subzones. That is, parent zones have all the information needed to
access servers for their children zones. The NS RRs that name the
servers for subzones are often not enough for this task since they
name the servers, but do not give their addresses. In particular, if
the name of the name server is itself in the subzone, we could be
faced with the situation where the NS RRs tell us that in order to
learn a name server's address, we should contact the server using the
address we wish to learn. To fix this problem, a zone contains "glue"
RRs which are not part of the authoritative data, and are address RRs
for the servers. These RRs are only necessary if the name server's
name is "below" the cut, and are only used as part of a referral
The idea that glue is "required" at the registry/registrar level is a widespread fallacy, mostly emerging from the the days when there were a scant few gGTLDs to choose from. As the number of TLDs to choose from has steadily increased over the years, it's becoming increasingly more common to encounter glueless TLD referrals. Take
goo.gl for example:
$ dig @d.nic.gl +noall +authority +additional goo.gl
goo.gl. 86400 IN NS ns4.google.com.
goo.gl. 86400 IN NS ns3.google.com.
goo.gl. 86400 IN NS ns1.google.com.
goo.gl. 86400 IN NS ns2.google.com.
Note the conspicuous lack of ADDITIONAL data here.
gl does not not need to provide glue for anything inside of
com. When this scenario is encountered, it becomes necessary to recurse through the
com TLD in order to obtain the IP address of one of those nameservers. Any glue encountered along the way will be followed as normal. I won't quote RFC 1034 §4.3.2 here as it's somewhat silly to restate basic operation fundamentals.
In short, glue is only mandatory where both the standards or the policy of specific registrars (or registries) require it. The latter requiring it is not necessarily an indication of the former. In cases where a glueless delegation from the TLDs is encountered, it's simply business as usual.
If glue were to be missing in a scenario where it is required (your actual question I suspect), and no other nameservers were available to provide the authoritative response, that would be a clear case of
NXDOMAIN would imply that an authoritative server could be reached, which is not the case here. The server which defines the zone cut is not authoritative for data beneath the cut (RFC 2181 §6.1), and an inability to reach the authoritative servers means that you've arrived at a dead end.