Veeky Forums could medieval trench warfare ever happen...

This. "Trench Warfare" was basically a 20th century variant on the good old-fashioned siege. Trench warfare is necessitated only be primacy of ranged weapons, which requires that cavalry be rendered wholly ineffectual (as the natural counter to missile weapons, generally). A castle makes cavalry worthless, so a siege is almost entirely missile-exchanges until infantry go "over the top" to attempt to storm with melee weapons. World War I went the way it did because cavalry became obsolete, and motorized vehicles were not yet capable of replacing them.

I still like to think many sailors last words were something to the tune of "What the fuck, WHY" when they had the misfortune of coming across Greek Fire.
They even made grenades with caltrops using the stuff. Can you imagine how fucking dangerous the stuff would have been if the method of making it was preserved long enough to be used with long range artillery.

Imagination

The term "last ditch effort" is literally rooted in medieval trench warfare in which literal ditches would be dug and fought in beyond main fortifications.

Short answer, yes it would just be absurdly uncommon

You may be misunderstanding.
The statement "we don't know how Greek fire was made" is not a claim that Greek fire was some secret super weapon.
Rather, we are able to make dozens and dozens of incendiary and exothermic weapons and warheads, any of which may be superior to Greek fire. We simply don't know what Greek fire actually was, and therefore have no way to conclusively prove WHICH formula it was.

It's not unlike the debate over linen armor. Various sources claim its use, by Alexander among others, yet we don't know how it was made. Same for many forms of leather armor. Boiled leather being the standout. We know it was a thing, we just don't know the specific process to make it.
We can make armor out of linen, we can make armor out of hardened leather. We just don't know if we're using THE method used by the ancients.
We cannot claim it is authentic therefore.

en.wikipedia.org/wiki/Battle_of_the_Trench

Not trench warfare in the traditional sense

Explain to me in a setting with early guns, why would swords and platemail still be used?

>early guns
Meaning what?
I feel the need to shortcut your post and ask if you are at all familiar with the pike and shot period, in which early guns explicitly were used in conjunction with swords and platemail.

It wasn't used for that long because it was a one-trick weapon. Once the musselmanns has figured out that you can put it out with vinegar and that the usual prevention measure (wet wood, untanned hides) do work, it quickly became too much of a hassle in face of dimishing returns.

The entire 15th century for starters, moron.

repeating crossbows.

Well, the first documented use of trenches was at the siege of Kaifeng in 1232
en.wikipedia.org/wiki/Mongol_siege_of_Kaifeng#Military_technology

Someone with a cavarly-poor army could create an extensive earthwork with trenches and thorn bushes (instead of barbed wire) to negate the enemy's horsemen. Say, dwarfs fighting centaur nomads. Such a system could be quite long and intrincate, akin to border defenses like:
en.wikipedia.org/wiki/Great_Wall_of_Gorgan
en.wikipedia.org/wiki/Roman_military_frontiers_and_fortifications
en.wikipedia.org/wiki/Derbent#History
en.wikipedia.org/wiki/Cilician_Gates
It would make more sense if it was a somewhat wide chokepoint in an otherwise impenetrable barrier, like a big mountain pass in a mountain range.

Dwarfs could use some mix of roman scorpio and chinese repeating crossbow that outranges the nomad bows. Centaurs could adopt the dwarf idea, using mercenaries/engineers/slaves to approach the enemy trench with a trench of their own, their purpose being to avoid being gutted by a javelin-sized projectile.

Meanwhile, the nomads go raid other places. Their servants keep the deadlock with the dwarfs. The latter get supplies from the nations that could be attacked if the dwarf fortifications were broken.

This state of affairs could last many years. Is this enough for you OP?

Forgot to add that the Iron Age already enables this idea, technology-wise. It's the other factors that are more random.

>platemail
What is it, the 19th century?