You can sort of add new keywords backwards-compatibly using a trick called "contextual keywords": you require that they be placed in a syntactic position in which no identifier could legally go, and you maintain them as legal identifiers for compatibility. C++ used this trick to introduce "final" and "override" by moving them before the opening "{".

You mean new reserved words? For example, I'm quite sure when C# added "record" it didn't break backward compatibility, as old code that uses "record" as a variable name still compiles.

Go has made changes like that by adding new predeclared identifiers ("any" is an example, I think?) but there's a distinction between predeclared identifiers and keywords.