It's got more attention now. There is an article on The Verge and it also references this long thread of ours in this forum.
One user in the Facebook comments listed a tedious work-around that has been working for him [paraphrased below]
"Before you leave your text messaging chat window, always back out to the main chat window so that you don't leave the chat window open on any one particular contact. It should be left open in the main chat window that lists all of your text messaging conversations."
Having to remember to do this is on the same level of killing the chat window after every text (the other work-around that has worked) but at least this bug is getting more and more widespread attention. Link to the article is below.
https://www.theverge.com/2020/12/11/22169881/apple-iphone-ios-14-text-messages-no-notifications-bug?fbclid=IwAR2ZaTdRdFh4N9MieLfWEcwJK_kwhR4tsE-CbAouGRLJ3JyuaRlo6fWQszo