Even though it is possible to have both install.rdf and manifest.json files in your extension, so you could release a version compatible with Thunderbird 60 and 68, it is not suggested for the following reasons:
The amount of changes is huge and some changes are incompatible with Thunderbird 60 so it will require extra steps to ensure the modified version still runs with Thunderbird 60.
You may actually break your add-on for Thunderbird 60 users by releasing a backward compatible version for Thunderbird 68 ("Do not fix something, that is not broken").
We think the time and resources needed to code and test backward compatible add-ons is not justified by the small amount of users running older versions of Thunderbird.
If you do want to stay backward compatible, you will find useful information here.
It is possible to maintain a legacy version and a WebExtension version of your add-on in parallel on ATN! You just need to use a higher major version number for the WebExtension version of your add-on and keep the old major version number when releasing a new legacy version. Basically releasing them in two different branches.