[CF-metadata] [cf-convention/cf-conventions] Clarify and update CF rules for deprecating content (#328)

The paragraph in the [CF rules document](https://urldefense.us/v3/__https://cfconventions.org/rules.html__;!!G2kpM7uM-TzIFchu!kARlSi-vy4vYD0KbbiHHsnruaUd23F_K5Va4APhRMDTprFm_kK76dOEvNF3CE-riPCTxVm7qfNw$ ) that discusses deprecation is focused on recent (or even the most recent) changes and

Re: [CF-metadata] [cf-convention/cf-conventions] Correction to the definition of "ocean sigma over z coordinate" in Appendix D (#314)

Hi all - The paragraph in the [rules document](https://urldefense.us/v3/__https://cfconventions.org/rules.html__;!!G2kpM7uM-TzIFchu!ka812U3rXTGiXhFTJ9vi-8Qw_cLVDhuM3XUJIQUi2EWbPJthBpBBagU3XRf6sgpMrh4w-Uane_Q$ ) that mentions deprecation seems focused on recent (or even the most recent)

Re: [CF-metadata] [cf-convention/cf-conventions] Correction to the definition of "ocean sigma over z coordinate" in Appendix D (#314)

> I would have thought that the the creation of all new CF-1.8 datasets should > be deprecated. Just listening in here + heard something that affects us, (i.e

Re: [CF-metadata] [cf-convention/cf-conventions] Correction to the definition of "ocean sigma over z coordinate" in Appendix D (#314)

Dear Jonathan, Oh - what a difference a day makes! Thanks for the text. I think it is fine, but it states that it is still OK to produce CF-1.8 datasets if they don't contain this particular formula - is that what we want? I would have thought that the the creation of all new CF-1.8 datasets