Definitely worth checking out :). At some point, they bit the bullet and included a programming language inside[0].

https://plantuml.com/preprocessing

It's enough to make it easy to organize common chart elements into libraries, and even create some DSL here and there - the latter being helpful if you're using PlantUML for note taking or in lieu of doodling :).

For instance, I got a very good mileage out of "trivial" definitions like:

  !procedure call($source, $target, $call)
  $source -> $target: $call
  activate $target
  !endprocedure

Forming a mini-DSL that let me turn notes like:

  call("Someone", "SomeoneElse", "FooBar()")
  asynccall("SomeoneElse", "SomeService", "Run(a=42)")

Into a rather large sequence diagram that's also readable as text, and possible to auto-generate to some extent.

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[0] - Which looks like a spawn of an unholy marriage between PHP and C preprocessor... but hey, it's a programming language.