I'm wondering why there's no release version of Thunderbird newer than 60.9.1
I know that there's a PortableApps version of 73.0 that's available, but that's a beta. At this writing, the most recent released version of Thunderbird is 68.4.2.
Thunderbird releases follow the path of Firefox Extended Support Release (ESR) versions, and I know that there are PortableApps releases of the current version of Firefox 68.x ESR. Although Thunderbird does two versions, the most current version never emerges from beta, and only the version that corresponds with Firefox ESR is actually released. With Thunderbird, beyond the Firefox ESR limitation of only security fixes, in the betas, a significant portion of extensions are non-functional.
The next full release of Thunderbird won't be until Firefox does its next cycle (78.0, I think) scheduled for June.
Because 72, 73, 74 are all betas, there really isn't widespread reason for any other than developers to be using them. At the same time, support for 60.9.1 has ended, and there are security fixes to 68.1, 68.2, 68.3, etc. that are not available to users that should not be running the beta versions.
May I suggest focusing on the version currently available, as a higher priority than implementing the betas?