I had this problem the temporary solution I found which I have used is to close any popup telling you to update Messenger. Open my browser (Chrome for me) and search for messenger (or click icon if shown). Click on Messenger and you can login as usual. Sometimes it fails login but just open a new tab and it works. I then input my code to load all messages and then everything works as before. Slightly tedious but hopefully they will realise their mistake (possibly after being fined) and will stop this protectionist policy. If not many people (including me) will be seriously looking for an alternative.
If anyone has found an alternative which can retain historical posts please share.