Your best way forward is to replace your widgets with the Plasma 6 versions. If you cannot replace a particular widget, then you’re out of luck, I suppose. That said, a quick search with plasma 6 compatible widgets (or similar) as the search term might reveal some decent replacements.
I saw earlier mentioned that only Plasma 5 widgets are showing in get new results. I’m sure that will be updated soon enough; it’s a question of patience.
No, you should not install an older version of Manjaro and stop updating. 
If you really have to ask that, all I can offer in return is:
If you wish to pursue this further; for example, if you would like some assistance or advice with updating specific widgets, please start (your own) new thread, and detail exactly what you hope to achieve. Doing this will allow others to help more effectively.
Cheers.