I have not been following the custom run announcements, but if you put "MTH custom run" into the search it has yielded quite a few announcements, especially from Mr Muffin's Trains and Berwyn's Toys and Trains. You can change the sorting from "Relevance" to "Date" to get the most recent announcements at the top. I know I have had my share of challenges with using the search feature, but I was able to get some relevant results.
The MTH Products board here also has many (if not all) of the custom run announcements from dealers.
The last option I know of (and may be the closest to a list of custom run offerings) is to go to MTH's official forum account and look under "Posts" (see this link: https://ogrforum.com/member/mth/posts). If MTH posts about a custom run (or anything else) anywhere on the forum, it should appear in their list of posts.
I realize that none of these are exactly what you're looking for, but I hope they get you close enough 
Edit:
@paulp575 posted:Here's the web page that shows all the 2021 items thus far: https://mthtrains.com/prod-sea...field_catalog%3A7742 and here's the link to 2022 items: https://mthtrains.com/prod-sea...field_catalog%3A7807
Looks like someone found a better solution! 
