Turboprops
Some turboprops have independent speed control (via a separate lever : the condition lever is sometimes used as an rpm demand input e.g. PT6?); some have have speed demand as a function of power lever position (speed demand comes via an 'interconnect' characteristic e.g. Dart, Tyne); some are constant speed (T56) as discussed above; some have different speed demanded depending on flight mode (e.g. TP400).
Turbojets/fans
Not really the subject of the question , agreed: rpm is variable - but NL (N1) depending on engine type, can be fairly linear with thrust e.g. mixed turbofan.
EPR
I think cold EPR and combined (integrated) hot+cold EPR have been used on some types. Some issues with icing and system complexity though ...