Pprune Radar - that ICAO reference is again in CAT context, not GA.
Sorry, but you're wrong. The ICAO standards don't specify that they only apply to CAT, otherwise as a GA pilot you could ignore all the airspace classifications, rules of the air, RT phraseology, equipment specifications, international licencing rules, etc, etc. The standards are published so that States may meet an international standard and so that industries throughout the world can provide equipment which meets a common standard and not one which operates in isolation. States may file differences to ICAO reccomendations, but they should then appear in their AIP. I can't find one which says that Airborne Collision Avoidance System (ACAS) standards only apply to CAT in the UK ... but then that's because they don't.
Bear in mind ICAO does not say that TCAS is the only show in town for collision avoidance systems (generically known as ACAS). Any company could produce a collision avoidance system which is not TCAS, but if they want it certificated for airworthiness purposes, then they'll almost certainly have to meet the ICAO Standards since this is what most States in the world base their requirements on.
TCAS standards take the generic ICAO ones and apply more detail to specify a common standard for people who build the TCAS equipment and operate collision avoidance systems using TCAS as their tool. A group called the RTCA publish those but they will still meet the ICAO specifications as a minimum, and do. These standards don't say they apply to CAT only. Why would they ??
As others have said, there are other systems out there such as FLARM and TCAD. Others are on the way too, like ADS-B. They still all find their specifications coming down from ICAO, and then being further detailed in RTCA or EUROCAE documentation, regardless of what type of aircraft they will be fitted in.
Also nobody is suggesting making TCAS mandatory; for GA that would be silly. The only bit which could be mandatory is transponder usage, unless technically impossible.
I don't believe anyone here
did say make TCAS mandatory ... or maybe I missed it.
Mandatory Mode S is currently the European administrators answer to everything. Many are still to be convinced, me included.