TB 115 supports 10.12 and higher, 128 supports 10.15 and higher. TB 115 is available here:

https://archive.mozilla.org/pub/thunderbird/releases/115.18.0/mac/

Yes, I have ESR 115.18, but I was wondering if T-bird ESR updates were being released as is being done on this schedule https://whattrainisitnow.com/release/?version=esr

I've also added a screenshot of the particular section.

The picture shows the schedule for Firefox. There is no version of TB above 115 that supports OS X 10.13.

https://www.thunderbird.net/en-US/thunderbird/releases/

I was using Firefox ESR's new schedule as an example. Firefox ESR updates had originally ended but Mozilla decided to add at least 6 more months of updates Firefox ESR 115.28 through 115.33. The question is if something similar either HAD happened or WAS going to happen with T-bird ESR 115.18.

Apparently you are as in the dark as I am about the possibility. Hopefully someone more knowledgeable than both of us might chime in here.

If the Thunderbird community was going to keep the old Tb 115 ESR updated with security and stability updates then they would have continued with updates past the Tb 115.18.0esr.

Mozilla Thunderbird is developed by MZLA Technologies Corporation, which is a wholly-owned, for-profit subsidiary of the Mozilla Foundation.

Firefox 115 ESR was originally going to end as of Fx 115.15.0esr but then Mozilla decided to continue extensions of updates since as this time is the third updates extension.