PL/I was infamous for it being possible to express valid code that read "IF IF THEN THEN ELSE ELSE".

Yep. One of the goals was that there would be no reserved words. Not sure why.

    IF IF = THEN THEN THEN = ELSE ELSE = END END

C++ has some contextual keywords as well:

  final (C++11)
  override (C++11)
  import (C++20)
  module (C++20)

But did they make those contextual in order to avoid conflicts with code that had existing identifiers before the new keyword was specified?

Yes.

C# is probably the king of contextual keywords, they have tens of them. They started I believe with `yield return` and `yield break` and then added all of the LINQ ones (select, from, where etc), and kept adding others (notnull, record, required, etc).

Java too:

> A further ten character sequences are restricted keywords: open, module, requires, transitive, exports, opens, to, uses, provides, and with. These character sequences are tokenized as keywords solely where they appear as terminals in the ModuleDeclaration, ModuleDirective, and RequiresModifier productions (§7.7).

https://docs.oracle.com/javase/specs/jls/se12/html/jls-3.htm...