I just tried to compile a C program with a GNU compiler version 4.9.2. The source code contained a few for int i=0; ... statements and the compiler gave me an error and indicated that I should use -std=c99 in order to compile for loop initial declarations. Apparently, such declarations were not valid before C99.
On an another machine, I have a more recent GNU compiler (8.1.1) where I can compile the same source code without explicitly specifying -std=c99.
Because GNU obviously made their compiler C99 compliant between 4.9.2 and 8.1.1, this lead me to the question if a recent C standard specified that a compiler has to adhere to C99 (or another standard).
Choosing whether to adhere to the C standard, or a particular version of it, is voluntary. The choice does not come from the C standard. It comes from outside. Anybody who makes a C implementation decides whether to conform to the 2018 C standard (or to conform mostly but not completely), whether to conform to the 2011 C standard, whether to conform to some “K&R” notion of C, or something else. Nothing in the C standard says, well, if you are conforming to this standard, your compiler has to conform to some previous version. The standard cannot actually require you to do anything until you choose to conform to the standard.
The C standard and the people who make it and the standards organizations that endorse and publish it have little power to make anybody do anything. They cannot publish the C standard and say you, René Nyffenegger, must obey the 2018 C standard. They are not law-making bodies. There are contracts between private parties which say some project will be produced in accordance to this standard or that standard, but those are private agreements, not public law.
In the 2018 C standard, paragraph 8 of the Foreword says:
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions related to conformity assessment, as well as information about ISO’s adherence to the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see the following URL: www.iso.org/iso/foreword.html.
Nor can the standard organizations prohibit you from writing a C compiler that does or does not conform to any particular version of the standard, nor from writing a compiler that conforms largely but not completely.
If you use the name of the C standard commercially, perhaps by claiming conformance to it, the standards organizations might have some legal rights in that regard. That involves international law and the law of many jurisdictions, which I cannot speak to authoritatively. I have not heard of any problems occurring from somebody claiming to conform to the C standard.
The standards organizations do officially withdraw old versions of the standards when a new version is published. This does not prevent you from writing a C implementation that conforms to an old version, but it would prevent you from claiming you are conforming to the current version when you are not. (For example, if a contract you agreed to required you to conform to the current C standard, that would change when the organization publishes a new version and withdraws the old one.)