From an implementation/SAP perspective, I am just wondering if are we supposed to keep our existing RFC objects since they are still supported in the cloud? Or by right, as per SAP's guide, we should be converting all these to oData services. I am just wondering how did other companies approach it (or even SAP's approach).
I saw some people quoting some SAP notes about discouraging use of RFCs, but all of those notes were already removed (wondering why).
I don't have the definitive answer, but I do remember those old notes, I also don't know why they were removed.
As I recall, the use of RFCs (and IDOCs, etc) were limited to a limited number of white-listed ones. There were a few manufacturing ones I recall available for integrating third party MES for example. The main focus to allow them was also limited to SAP to SAP communication (Like to Onprem).
Looks like now you can develop them with Developer Extensibility per your link, but looks like there are also restrictions, for example, that comm arrangement is for use only with a Cloud Connector, which implies connection to OnPrem systems.
The 5 golden rules for the Cloud Mindset (https://blogs.sap.com/2022/08/31/the-cloud-mindset/) include one of the Golden Rules to use "cloud-like" integrations - which RFCs are not really considered to be Cloud like (access over http, etc). The SAP Note (https://me.sap.com/notes/2920697/E) is deprecated and was specific to extended edition. There are links to other replacement notes in that note, might be worth it to review those (like the one for ABAP Code checks).
So the recommendation I am aware of would be to consider replacing your RFCs with Odata services.
I hope this helps.