What is the best word or adjective for someone who does not meet the deadline for delivery?

laggard (Collins)

a person who lags behind

Use case:

… Mr. Flaherty told reporters Tuesday that all sides agreed to the deadline and an extension for laggard projects simply wouldn't be fair.
("Flaherty refuses to extend stimulus deadline for laggards," The Globe and Mail,)

I'd use the compound noun "deadline offender" as it seems fairly weell self-defined and as, also, it a communicates formally the idea of infraction to a rule, or of infringement to a convention, to an agreement.

I might use the term "chronic procrastinators" or the alliterative "procrastinating programmers."

Behind deadline

The problem here, is not really ‘the programmers’ but ‘the person managing them’.

Programmers are by nature people who love to delve into problems and fix and fiddle about with things.

Generally they are not malicious in this but instead need someone to ‘tell them when to stop!’ - so that the software release can Actually Go Out - at some point.

The other thing that goes on, is that delving into problems often Uncovers Other Deeper Problems - which may even be organisational, business-wide or anyway, more complex or serious in nature - leaving us with - a ball of spaghetti!

So I would call them ‘unmanaged tinkerers’.

If you Reaaally feel they are malicious, then how about:

• System laggards
• Tardy system fixers
• Programmatic philanderers
• Programming wastrels
• Meddling meanderers
• Lazy programmers

But if you truly want to get them to hurry up, stop criticising them, and instead set rewards - and praise Johnny, Fred, and Jane who get mentioned and special thanks for ‘being on deadline’.

Otherwise - you are ‘feeding the problem’ - not ‘the solution’.

https://dictionary.cambridge.org/dictionary/english/deadline

A less derogatory term for somebody who ignores externally-imposed constraints is “ivory tower”:

Cambridge English Dictionary:

To live or be in an ivory tower is not to know about or to want to avoid the ordinary and unpleasant things that happen in people's lives:

• Academics sitting in ivory towers have no understanding of what is important for ordinary people.

Lexico:

A state of privileged seclusion or separation from the facts and practicalities of the real world.
‘the ivory tower of academia’

American Heritage Dictionary:

A place or attitude of retreat, especially preoccupation with lofty, remote, or intellectual considerations rather than practical everyday life.

I couldn’t find a definition that gave any part of speech other than noun, but it would probably be understood if used as an adjective:

These ivory towers programmers want to get their software perfect, and don’t recognize the concept of “good enough to ship”.

Programmers who don't meet their (boss's) deadlines are unreliable.

• adjective: an unreliable programmer
• noun: programmer's unreliability

It all depends whether the deadline is a hard or soft one. If it's a hard deadline, 90% done is the same as 0% done since it can't be delivered. If it's a soft deadline, the work can be finished and submitted later.

• adjective: an slow programmer
• noun: programmer's slowness

Other words can be used: late/lateness, tardy/tardiness, unpunctual/unpunctuality although they can mean arriving late to work rather than submitting work late.